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Effect of NiO addition on oxygen reduction reaction at lanthanum strontium cobalt ferrite cathode for solid oxide fuel cell
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e9
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Effect of NiO addition on oxygen reduction reaction at lanthanum strontium cobalt ferrite cathode for solid oxide fuel cell Mubashar Nadeem a, Bobing Hu a, Changrong Xia a,b,* a
CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering & Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, No. 96 Jinzhai Road, Hefei, Anhui Province, 230026, PR China b Energy Materials Center, Anhui Estone Materials Technology Co. Ltd, No.12 Weigang Road, Bengbu, Anhui Province, 233400, PR China
article info
abstract
Article history:
Lanthanum strontium cobalt ferrite (LSCF), a perovskite's family member, has gained much
Received 11 January 2018
attention as the electrocatalyst for the oxygen reduction reaction (ORR) in intermediate
Received in revised form
temperature solid oxide fuel cells. However, it still needs a strategy to improve its catalytic
6 March 2018
activity. In this work, NiO is primarily investigated as a possible synergistic catalyst for ORR
Accepted 8 March 2018
on the LSCF surface. The effect of NiO particles on the effective oxygen chemical surface
Available online xxx
exchange coefficient is revealed with the electrical conductivity relaxation (ECR) technique. At 800
C, the coefficient is increased from 3.48 105 to 6.65 105 cm s1 and
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cm s1 when NiO particles are deposited using the sputter and drop coating
Keywords:
6.9 10
Solid oxide fuel cell
methods, respectively. Adding 5 wt % NiO to LSCF reduces the area specific interfacial
Lanthanum strontium cobalt ferrite
polarization resistance, for example from 0.108 to 0.082 U cm2 at 700 C, as demonstrated
Oxygen reduction reaction
by impedance spectroscopy on symmetric cells using samaria-doped ceria as the electro-
Interfacial polarization resistance
lyte. Adding NiO can also improve the performance of anode supported button cells,
NiO
increasing the peak power density from 0.731 to 1.031 W cm2 at 800 C. On the whole, the increased oxygen surface exchange rate together with the reduced electrode resistance and improved power density, exhibit that NiO is a potential additive to enhance the LSCF catalytic activity. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Efficient, clean and direct conversion of chemical energy into electrical energy can be achieved by the usage of solid oxide fuel cells (SOFC), where oxygen molecules from air are reduced to oxygen ions at the cathode and subsequently
transported to the anode to oxidize the fuel molecules such as H2 and CO. The oxygen reduction reaction (ORR) is believed to be the main contributor towards the electrode polarization resistance. Thus, the highly electrocatalytically active cathodes, especially those which operate at an intermediate temperature range of 600e800 C, have attracted
* Corresponding author. CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering & Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, No. 96 Jinzhai Road, Hefei, Anhui Province, 230026, PR China. E-mail address: [email protected] (C. Xia). https://doi.org/10.1016/j.ijhydene.2018.03.053 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Nadeem M, et al., Effect of NiO addition on oxygen reduction reaction at lanthanum strontium cobalt ferrite cathode for solid oxide fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.053
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much attention over the past years [1,2]. In particular, LaxSr1xCoyFe1yO3-d (LSCF), a member of the perovskite's family, has gained tremendous popularity as the cathode material for intermediate-temperature applications due to its relatively high ionic and electronic conductivity compared with other types of oxide electrocatalysts. Besides being an ORR electrocatalyst, LSCF offers high electrode performance by facilitating a bulk pathway for oxygen anion transport [3e6]. However, despite these benefits, there is hindrance for its practical application as the ORR on the LSCF cathode is limited by the surface exchange process rather than the bulk charge transport process, and it is the cathode reaction which dominantly contributes towards SOFC performance [7e9]. A number of strategies have been adopted in order to enhance the surface reaction kinetics. Initially, noble metals like Pd [10e12], Rh [11], and Ag [13e16] were utilized to enhance the ORR on LSCF because of their high catalytic activity. The LSCF cathode surface was modified by nanoparticles of these metals through the infiltration process. And, the performance had been improved, for example, by the reduction in the area specific interfacial polarization resistance (ASR) at 700 C from 0.14 U cm2 for bare LSCF to 0.044 U cm2 with infiltration of 7.5 mg cm2 Pd, corresponding to an improving factor of 3.2 [12]. However, the noble metals were often avoided due to their high cost. They were replaced with doped ceria materials with the belief that ceria can boost the ORR kinetics by enhancing the surface exchange process as well as increasing the oxygen ionic conductivity. In fact, in comparison with the bare LSCF cathode, Nie et al. [17] did achieve a considerably lower ASR of 0.40 U cm2 at 650 C by infiltrating samaria-doped ceria into the porous LSCF scaffolds. Similarly, Chen et al. [10] demonstrated that upon infiltration of 160 mL of 0.25 mol L1 gadolinia-doped ceria, a polarization resistance of 0.10 U cm2 at 650 C can be obtained. Furthermore, mixed ionic and electronic conductors such as Sm0.5Sr0.5CoO3-d [18], La0.6Sr0.4CoO3-d [19,20], and Pr0.6Sr0.4CoO3-d [21], which are good electrocatalysts for ORR, were also employed to enhance the ORR kinetics. For example, Lou et al. have reported that infiltration of Sm0.5Sr0.5CoO3-d into LSCF cathodes can reduce ASR from 0.103 to 0.036 U cm2 at 750 C, along with an ~22% increment in the peak power densities [18]. In addition to the aforementioned strategies, the ORR on the LSCF cathode has been significantly improved by some rare earth and transition metal compounds, which are neither electronic/ionic conductors nor electrocatalysts. For example, Hong et al. [22] has reported an improving factor of 2.35 at 700 C by infiltrating 8.30 wt % BaCO3. They further demonstrated that coating 0.85 mg cm2 BaCO3 onto the surface of LSCF can increase the kchem of LSCF from 5.2 105 to 75 105 cm s1 at 800 C. Similarly, SrCO3 can also increase the ORR activity by an improving factor of 1.94 at 700 C [23]. Additionally, Zhang et al. [24] have shown that deposition of CaO particles can also increase the kchem. Similarly, the kchem is increased by a factor of about 3 at 750 C with the coating of CuO, as reported by Hong et al. [25]. Recently, Yang et al. reported that the solution deposition of MgO particles on the surface of LSCF can increase the kchem by a factor of up to 2.4 at 750 C [26]. However, Wang et al. [27,28] reported a discrepancy with transition metal compounds and found that
deposition of Cr2O3 on the LSCF surface results in the deterioration of the ORR activity. In this communication, we report the findings on the effect of NiO, a p-type semiconducting material as well as a transition metal compound, on the ORR activity of LSCF. Kinetics of the oxygen reduction reaction were explored with the electrical conductivity relaxation (ECR) technique to determine the NiO effect on kchem. The ORR activity was further evaluated by using symmetric and single cells.
Experimental Powders synthesis La0.6Sr0.4Co0.2Fe0.8O3d (LSCF) perovskite powders were prepared by the citric-EDTA method [29]. Stoichiometric amounts of metal nitrates La(NO3)3.6H2O (99.99%), Sr(NO3)2 (99.5%), Co(NO3)2.6H2O (98.5%) and Fe(NO3)3.9H2O (98.5%) to the nominal composition were dissolved in distilled water to make a solution. Citric acid (99.5%) and EDTA (99.5%) were subsequently added with a molar ratio of citric-acid: EDTA: metalions ¼ 1:1:1. The pH was maintained at ~7.0 with ammonia solution. The LSCF solution was then heated on a hot plate till self-combustion, forming the primary black powders. The assynthesized powders were subsequently calcined at 800 C for 2 h to remove the carbon residue and to obtain fine LSCF perovskites. The glycine-nitrate method was used to prepare Sm0.2Ce0.8O1.9 (SDC) electrolyte [30] and NiO powders, followed by heating the ashes at 600 and 850 C for 2 h, respectively. The NiO and LSCF powders were mixed at the weight ratio of 1:1 and sintered at 1000 C for 10 h to check the chemical compatibility. All the chemicals were purchased from Sinopharm Chemical Reagent Co. Ltd., China.
Cell fabrications Symmetrical cells were composed to evaluate the effect of NiO on the interfacial polarization resistance. Disc-shaped SDC pellets were prepared by uniaxially pressing the SDC powder at 250 MPa using a 13 mm diameter stainless steel die, followed by sintering at 1350 C for 5 h to form dense SDC pellets as the electrolyte substrates. To fabricate LSCF cathode, LSCF slurry was prepared by mixing the LSCF powder with an organic binder (a-terpineol as solvent and ethyl cellulose as the binder). The slurry was then screen-printed onto both sides of the SDC pellets and heated at 1000 C for 2 h to form symmetrical cells. To design composite cathodes, NiOLSCF composite powders with 5 and 10 wt % NiO were prepared by adding appropriate amounts of Ni(NO3)2.6H2O to the LSCF solution, followed by heating and calcination processes, the same as preparing the LSCF powders. The designing process of 5 and 10 wt % NiO-LSCF composite cathodes was also same as for the bare LSCF cathode. The electrochemical performance was further evaluated using anode-supported single cells with yttria-stabilized zirconia (YSZ), obtained from Sinopharm Chemical Reagent Co. Ltd., China, as the electrolyte, Ni-YSZ as the anode and SDC as the interlayer between YSZ and LSCF. Firstly, NiO-YSZ anode-support was tap casted and pre-fired at 900 C for 2 h. Then, an active
Please cite this article in press as: Nadeem M, et al., Effect of NiO addition on oxygen reduction reaction at lanthanum strontium cobalt ferrite cathode for solid oxide fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.053
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NiO-YSZ layer and YSZ electrolyte were deposited onto the anode support by a particle suspension coating process followed by co-firing at 1400 C for 5 h. To prevent the reaction between LSCF cathodes and YSZ electrolyte, SDC buffer layer was then applied on the YSZ surface and fired at 1200 C for 2 h. The cathodes were fabricated in the same manner as the symmetrical cells.
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was obtained by heating the mixture of LSCF and NiO in the weight ratio of 1:1 at 1000 C for 10 h. Fig. 1a indicates a well fitted single phase perovskite structure of LSCF with cell parameters a ¼ b ¼ 0.5475 nm, c ¼ 1.347 nm, a ¼ b ¼ 90 and
Electrical conductivity relaxation measurement The electrical conductivity relaxation (ECR) technique was used to demonstrate the effect of NiO particles on the surface oxygen reduction kinetics. The dense rectangular bars were prepared by pressing the LSCF powders at 300 MPa and sintering at 1400 C for 5 h in air [22]. The sintered bars, with size dimensions 32 5.48 0.60 mm3, had relative density greater than 97% of the theoretical value measured by the Archimedes method. Two methods were adopted to grow NiO particles on the bar surfaces. In the first method, the 0.05 ml, 0.10 ml, 0.15 ml and 0.20 ml Ni(NO3)2.6H2O solutions, with the concentration of 0.3 mol L1, were drop coated onto the bar surfaces, half volume per each of the two bar sides. This was followed by heating the bars at 800 C to form the NiO particles on the bar surfaces. In the second method, bar surfaces were coated with nickel films using sputter deposition (JFC-1600, JEOL). The coating was carried out for 60 s on both sides of the bar at 20 mA under a pressure of 10 Pa in air. This was followed by heating the Ni film in nitrogen atmosphere at 800 C for 2 h to form Ni particles, and then subsequently in air at 800 C for 2 h to form NiO particles. The conductivity was measured using the four-probe method with a digital multimeter (Keithley, 2001-785D). The gas atmosphere was abruptly changed from oxygen partial pressure PO2 ¼ 0.21 atm (O2 þ N2, Nanjing special gas Factory Co., Ltd.) to PO2 ¼ 1 atm (O2). The total gas flow rate was maintained at 200 ml min1 to ensure that the PO2 of the gas achieved equilibrium in no more than 20 s.
Sample characterization The XRD patterns were obtained using the technique of X-ray diffraction (XRD, Rigaku TTR-III diffractometer) with a Cu-Ka radiation source in the range of 20e80 . The morphologies were characterized using scanning electron microscopy (SEM, JSM-6700F). Electrochemical measurements were conducted with a Zahner Im6e electrochemical workstation. The impedance of the symmetric cells was measured in ambient air, and the frequency ranged from 101e106 Hz with AC signal amplitude of 10 mV. The performance of the single cell was evaluated using humidified (~3% H2O) hydrogen as the fuel, with a flow rate of 25 ml min1, and ambient air as the oxidant. All impedance spectra for single cells were recorded under open-circuit conditions with 10 mV AC signal perturbation.
Results and discussion Compatibility between LSCF and NiO Fig. 1aec shows the rietveld refinement analysis of LSCF, NiO and LSCF-NiO composite, respectively. The composite powder
Fig. 1 e The Rietveld refinement analysis of (a) LSCF powder, (b) NiO powder, and (c) LSCF-NiO composite powders heated at 1000 C for 10 h.
Please cite this article in press as: Nadeem M, et al., Effect of NiO addition on oxygen reduction reaction at lanthanum strontium cobalt ferrite cathode for solid oxide fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.053
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g ¼ 120 . In LSCF-NiO composite powder (Fig. 1c), well fitted peaks are distinguishable and can be assigned to either LSCF or NiO. The cell parameters of LSCF in the composite powder are a ¼ b ¼ 0.5471 nm, c ¼ 1.349 nm, a ¼ b ¼ 90 and g ¼ 120 . No impurity peak was observed. The peak positions and cell parameters do not change in rietveld refinement analysis of composite powder, which confirms the good compatibility between LSCF and NiO under the testing conditions.
NiO effects on the surface exchange rate The NiO effect on the oxygen surface reduction rate was investigated using LSCF bars coated with various amounts of NiO particles via solution deposition method. Fig. 2 displays a surface overview of bare LSCF and NiO coated LSCF samples. Fig. 2a displays a dense LSCF surface, the grain is up to 10 mm in size and the grain-boundaries are clearly presented. The relative density of LSCF bars was found to be more than 97%, as determined by the Archimedes method in distilled water. In Fig. 2b, fine NiO particles can be seen covering the LSCF surface. The particles must be NiO since they were obtained by heating the drop-coated Ni(NO3)2.6H2O solution. The fine particles are not so uniform and naked LSCF surface is observable at some places due to the low coating amount of NiO particles. In addition, the LSCF grain boundaries are observable. As the coating amount is increased, the NiO
particles form a porous monolayer in a more uniform manner (Fig. 2c and d, respectively). The greater coating amount increases the thickness of the monolayer such that some cracks develop in the surface as shown in Fig. 2e. NiO decoration effect on the oxygen surface exchange kinetics of LSCF was explored with the electrical conductivity relaxation technique. Fig. 3a shows normalized conductivity profiles and their fitting curves for bare and NiO decorated LSCF bars as a function of relaxation time at 800 C, with a PO2 step change from 0.21 to 1 atm oxygen. A good agreement was observed between experimental data and fitting curves. The bare LSCF sample takes about 6200 s to attain equilibrium. When LSCF is drop coated with 0.05 ml Ni(NO3)2.6H2O solution, the relaxation time is reduced to 900 s. In our study, the bulk transport properties are the same and the driving force conditions are also identical. Therefore, the reduction in the relaxation time can be purely attributed to the enhancement in oxygen surface exchange process associated with NiO particles. The LSCF samples display 600 and 470 s relaxation times for drop coating of 0.10 ml and 0.15 ml Ni(NO3)2.6H2O solution, respectively. However, the further drop coating of 0.20 ml Ni(NO3)2.6H2O solution leads to a relative increase in the relaxation time from 470 to 525 s. The 0.15 ml drop coating is thus optimal, when the oxygen surface exchange process occurs at its maximum rate. The relaxation time of 0.15 ml drop coated LSCF is 13% less in magnitude than the bare LSCF,
Fig. 2 e Surface SEM micrographs for (a) a bare LSCF bar and LSCF bars drop coated with Ni(NO3)2.6H2O solution of volume (b) 0.05 ml, (c) 0.10 ml, (d) 0.15 ml, and (e) 0.20 ml. Please cite this article in press as: Nadeem M, et al., Effect of NiO addition on oxygen reduction reaction at lanthanum strontium cobalt ferrite cathode for solid oxide fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.053
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surface in a discontinuous manner as shown in Fig. 4a. The NiO particles are grown on the LSCF surface by the sputter deposition method for 60 s. Compared to the bare LSCF, the relaxation time drops to 3400 s and kchem increases to 6.65 105 cm s1 at 800 C suggesting an improvement in the oxygen surface exchange process. Fig. 4b compares kchem for different samples, demonstrating that the surface exchange rate can be improved by both drop coated and sputter deposited NiO particles by a factor of ~20 and ~2, respectively. Nevertheless, in the case of bare LSCF, the oxygen surface exchange reaction takes place only on the surface of the bare LSCF i.e. at the LSCF-gas two phase interface. After NiO decoration, the oxygen exchange reaction can proceed not only at the LSCF-gas interfaces, but also at the LSCF-NiO-gas three-phase boundary. NiO decoration reduces the exposed surface area of LSCF to gas phase, while increases the length of the three-phase boundaries. Therefore, it is possible that the ORR can proceed more easily at the LSCF-NiO-gas threephase boundaries, hence the improved oxygen surface exchange kinetics.
Electrochemical performance of LSCF-NiO composite electrode The effect of NiO on the ORR of LSCF was also investigated with impedance spectroscopy. Fig. 5 shows ASR in the temperature range of 550e800 C for symmetrical cells composed of SDC electrolyte supported LSCF and LSCF-NiO composite
Fig. 3 e (a) Normalized conductivity data and fitting curves, and (b) chemical oxygen surface exchange coefficients (kchem) for LSCF drop coated with Ni(NO3)2.6H2O solution. suggesting a significant improvement in oxygen surface exchange rate. The chemical surface exchange coefficients, kchem (cm s1), were obtained by non-linear least squares fitting of the normalized conductivity profiles shown in Fig. 3a. At 800 C, the surface exchange coefficient of the bare LSCF is calculated to be 3.48 105 cm s1, which agrees well with the literature values in the range of 2.4e3.6 105 cm s1 at the same temperature [31e33]. Fig. 3b compares the kchem values at different temperatures. The increase in the drop coating volume of Ni(NO3)2.6H2O solution from 0.05 to 0.15 ml, increases kchem at 800 C from 2.97 104 to 6.9 104 cm s1, well above 3.48 105 cm s1 for the bare LSCF. The effective surface exchange coefficient is improved by a factor of ~20. Further drop coating of 0.20 ml volume, reduces kchem to 6.6 104 cm s1. Thus, 0.15 ml drop coating of Ni(NO3)2.6H2O solution is optimal for the maximum increase in kchem. The NiO effect on kchem was further investigated with LSCF bar deposited with isolated particles. NiO nanoparticles of about 300 nm in size are uniformly distributed over the LSCF
Fig. 4 e (a) Surface SEM micrograph for LSCF bar decorated with NiO for 60 s, and (b) chemical oxygen surface exchange coefficients (kchem) for sputter and drop coated LSCF bars.
Please cite this article in press as: Nadeem M, et al., Effect of NiO addition on oxygen reduction reaction at lanthanum strontium cobalt ferrite cathode for solid oxide fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.053
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Fig. 5 e Electrode area specific resistances (ASR) for LSCF and LSCF-NiO composite electrodes.
electrodes. The ASR for the bare LSCF electrode is 0.108 U cm2 at 700 C, which is lower than the previously reported resistance of 0.16 U cm2 at the same temperature [23]. The different fabrication procedures including powder synthesis, electrolyte sintering temperature and electrode heating conditions may cause the difference in microstructure and consequently the resistance. The incorporation of 5 wt % NiO into the LSCF, results in the reduction of the ASR from 0.108 to 0.0802 U cm2 at 700 C, corresponding to an improving factor of 1.35, justifying the enhancing properties of NiO towards the activity of the LSCF electrocatalyst. In contrast, 10 wt % NiO addition leads to the higher ASR of 0.157 U cm2 at 700 C, compared to the bare LSCF electrode. Shown in Fig. 6a are the impedance spectra for bare LSCF and LSCF-NiO composite electrodes measured at 700 C in air. The resistances of the electrolyte and lead wires were subtracted for the sake of better understanding the electrode resistances. Although the ORR at cathodes has discrepancies, however, it is generally considered that the reaction contains many elementary processes including oxygen gas diffusion, oxygen surface adsorption/dissociation and incorporation,
Fig. 6 e (a) Electrochemical impedance spectra at 700 C for symmetrical cells with bare LSCF and LSCF-NiO electrodes, (b) the equivalent circuit for fitting the impedance spectra, and (c) the high and low frequency resistance versus NiO content. Please cite this article in press as: Nadeem M, et al., Effect of NiO addition on oxygen reduction reaction at lanthanum strontium cobalt ferrite cathode for solid oxide fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.053
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and oxygen ion migration at the electrode-electrolyte interface [34e36]. The LSCF electrode reveals two depressed arcs in the impedance spectrum, a high frequency arc and a low frequency arc. ASR is obtained by calculating the difference in the high and low-frequency intercepts at the real axes. After NiO addition, the spectra show the same characteristics as bare LSCF, implying that there are two rate-limiting steps. To understand these steps, equivalent circuit of L1R0(R1/CPE1)(R2/ CPE2) was used to fit the spectra using Zview program, Fig. 6b [34,37]. In the circuit, L1 is the inductance connected in series with a resistive element R0, representing the resistance of
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electrolyte and lead wires, which is not shown in the impedance spectra. The circuit elements R1 and R2, parallel with a constant phase element (CPE), are resistances according to high and low-frequency arcs, respectively. Fig. 6c represents the variation in high and low frequency resistance upon NiO addition to LSCF electrode. The high frequency resistance R1 remains almost same suggesting that the process associated with R1 does not change with NiO addition. On the other hand, low frequency resistance R2 shows much change. For example, it decreases from 0.09 to 0.06 U cm2 at 700 C with 5 wt % addition of NiO. Maier et al. [38,39] and Liu et al. [8] have
Fig. 7 e (a) SEM crosssectional view of a single cell with four-layer structure using bare LSCF cathode, (b) enlarged view of the porous LSCF cathode, (c) cell voltage and power density as a function of current density measured at 800 C for single cells using LSCF, LSCF-NiO, LSCF-SDC, and LSCF-SDC-NiO cathodes, and (d) impedance spectra measured under open circuit conditions. Please cite this article in press as: Nadeem M, et al., Effect of NiO addition on oxygen reduction reaction at lanthanum strontium cobalt ferrite cathode for solid oxide fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.053
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correlated R1 with oxygen ion transfer across the electrodeelectrolyte interface, while R2 with oxygen surface exchange reaction. The reduction in R2 suggests that NiO has significant synergistic ORR activity and accelerates the oxygen incorporation process, agreeing well with the ECR results.
Single cell performance The electrochemical performance was further evaluated with anode-supported single cells. Fig. 7a represents the cross sectional view of an anode-supported single cell comprising Ni-YSZ anode, YSZ electrolyte of ~13 mm thickness, SDC buffer layer of ~3 mm thickness and porous LSCF cathode. Fig. 7c compares the current-voltage curves and the corresponding power densities when humidified hydrogen was used as the fuel and ambient air as the oxidant. The cell with single phase LSCF cathode shows a peak power density of 0.731 W cm2 at 800 C, which is comparable with previous experimental data [40]. The peak power density increases to 1.03 W cm2 when LSCF-NiO composite with 5 wt % NiO is used as the cathode. The positive effect of NiO on the electrochemical performance was also observed with LSCF-SDC composite cathodes. The cell with LSCF-SDC composite cathode performs with a higher peak power density of 0.837 W cm2, which is further increased to 0.858 W cm2 with the LSCF-SDC-NiO cathode. Fig. 7d displays impedance spectra measured at 800 C under open circuit conditions. The cell with the LSCF-NiO composite cathode gives the total interfacial polarization resistance of 0.124 U cm2, lower than the resistance of 0.222 U cm2 for the bare LSCF cathode. Similarly, the resistance for the cell with LSCF-SDC-NiO composite cathode is reduced to 0.144 from 0.164 U cm2 for the LSCF-SDC cathode based cell. The increment in peak power densities and reduction in total interfacial polarization resistances must derive from the enhanced electrochemical performance associated with NiO since the anode and electrolyte nature is same in all the cells.
Conclusion NiO has been investigated as the synergistic catalyst for the oxygen reduction reaction on LSCF. The decoration of NiO particles by the drop coating and sputter deposition methods result in the successful acceleration of the oxygen surface exchange rate, increasing the effective chemical oxygen surface exchange coefficient by a factor of ~20 and ~2, respectively. Adding 5 wt % NiO to LSCF can reduce the interfacial polarization resistance of symmetrical cells from 0.108 to 0.082 U cm2 at 700 C and increase the peak power density of single cells from 0.731 to 1.031 W cm2 at 800 C. Thus, NiO is a potential material which can be used synergistically to enhance the kinetics of the oxygen reduction reaction on the LSCF cathode.
Acknowledgements We are very grateful to the financial support from National Natural Science Foundation of China (91645101).
references
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[20] Lee S, Miller N, Abernathy H, Gerdes K, Manivannan A. Effect of Sr-doped LaCoO3 and LaZrO3 infiltration on the performance of SDC-LSCF cathode. J Electrochem Soc 2011;158:B735e42. [21] Lee S, Miller N, Staruch M, Gerdes K, Jain M, Manivannan A. Pr0.6Sr0.4CoO3d electrocatalyst for solid oxide fuel cell cathode introduced via infiltration. Electrochim Acta 2011;56:9904e9. [22] Hong T, Chen F, Xia C. Barium carbonate nanoparticle as high temperature oxygen reduction catalyst for solid oxide fuel cell. Electrochem Commun 2015;51:93e7. [23] Li M, Sun Z, Yang W, Hong T, Zhu Z, Zhang Y, et al. Mechanism for the enhanced oxygen reduction reaction of La0.6Sr0.4Co0.2Fe0.8O3d by strontium carbonate. Phys Chem Chem Phys 2017;19:503e9. [24] Zhang L, Hong T, Li Y, Xia C. CaO effect on the electrochemical performance of lanthanum strontium cobalt ferrite cathode for intermediate-temperature solid oxide fuel cell. Int J Hydrogen Energy 2017;42:17242e50. [25] Hong T, Brinkman K, Xia C. Copper oxide as a synergistic catalyst for the oxygen reduction reaction on La0.6Sr0.4Co0.2Fe0.8O3d perovskite structured electrocatalyst. J Power Sources 2016;329:281e9. [26] Yang Y, Li M, Ren Y, Li Y, Xia C. Magnesium oxide as synergistic catalyst for oxygen reduction reaction on strontium doped lanthanum cobalt ferrite. Int J Hydrogen Energy 2018;43:3797e802. [27] Wang CC, O'Donnell K, Jian L, Jiang SP. Co-deposition and poisoning of chromium and sulfur contaminants on La0.6Sr0.4Co0.2Fe0.8O3-d cathodes of solid oxide fuel cells. J Electrochem Soc 2015;162:F507e12. [28] Wang CC, Becker T, Chen K, Zhao L, Wei B, Jiang SP. Effect of temperature on the chromium deposition and poisoning of La0.6Sr0.4Co0.2Fe0.8O3-d cathodes of solid oxide fuel cells. Electrochim Acta 2014;139:173e9. [29] Martynczuk J, Arnold M, Wang H, Caro J, Feldhoff A. How (Ba0.5Sr0.5)(Fe0.8Zn0.2)O3ed and (Ba0.5Sr0.5)(Co0.8Fe0.2)O3ed perovskites form via an EDTA/citric acid complexing method. Adv Mater 2007;19:2134e40.
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[30] Ding D, Liu B, Zhu Z, Zhou S, Xia C. High reactive Ce0.8Sm0.2O1.9 powders via a carbonate co-precipitation method as electrolytes for low-temperature solid oxide fuel cells. Solid State Ionics 2008;179:896e9. [31] Hu B, Wang Y, Xia C. Oxygen incorporation at the threephase boundary of LSCFeSDC composite. J Power Sources 2014;269:180e8. [32] Hu B, Wang Y, Xia C. Effects of ceria conductivity on the oxygen incorporation at the LSCF-SDC-gas three-phase boundary. J Electrochem Soc 2015;162:F33e9. [33] Bouwmeester HJ, Den Otter M, Boukamp BA. Oxygen transport in La0.6Sr0.4Co1yFeyO3d. J Solid State Electrochem 2004;8:599e605. [34] Kim J-D, Kim G-D, Moon J-W, Park Y-i, Lee W-H, Kobayashi K, et al. Characterization of LSMeYSZ composite electrode by ac impedance spectroscopy. Solid State Ionics 2001;143:379e89. [35] Jørgensen MJ, Mogensen M. Impedance of solid oxide fuel cell LSM/YSZ composite cathodes. J Electrochem Soc 2001;148:A433e42. [36] Peng R, Wu T, Liu W, Liu X, Meng G. Cathode processes and materials for solid oxide fuel cells with proton conductors as electrolytes. J Mater Chem 2010;20:6218e25. [37] Jiang Z, Lei Z, Ding B, Xia C, Zhao F, Chen F. Electrochemical characteristics of solid oxide fuel cell cathodes prepared by infiltrating (La,Sr)MnO3 nanoparticles into yttria-stabilized bismuth oxide backbones. Int J Hydrogen Energy 2010;35:8322e30. [38] Baumann FS, Fleig J, Konuma M, Starke U, Habermeier H-U, Maier J. Strong performance improvement of La0.6Sr0.4Co0.8Fe0.2O3d SOFC cathodes by electrochemical activation. J Electrochem Soc 2005;152:A2074e9. [39] Baumann FS, Fleig J, Habermeier H-U, Maier J. Impedance spectroscopic study on well-defined (La,Sr)(Co,Fe)O3d model electrodes. Solid State Ionics 2006;177:1071e81. [40] Liu Z, Liu M, Yang L, Liu M. LSM-infiltrated LSCF cathodes for solid oxide fuel cells. J Energy Chem 2013;22:555e9.
Please cite this article in press as: Nadeem M, et al., Effect of NiO addition on oxygen reduction reaction at lanthanum strontium cobalt ferrite cathode for solid oxide fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.053
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Effect of NiO addition on oxygen reduction reaction at lanthanum strontium cobalt ferrite cathode for solid oxide fuel cell Mubashar Nadeem a, Bobing Hu a, Changrong Xia a,b,* a
CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering & Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, No. 96 Jinzhai Road, Hefei, Anhui Province, 230026, PR China b Energy Materials Center, Anhui Estone Materials Technology Co. Ltd, No.12 Weigang Road, Bengbu, Anhui Province, 233400, PR China
article info
abstract
Article history:
Lanthanum strontium cobalt ferrite (LSCF), a perovskite's family member, has gained much
Received 11 January 2018
attention as the electrocatalyst for the oxygen reduction reaction (ORR) in intermediate
Received in revised form
temperature solid oxide fuel cells. However, it still needs a strategy to improve its catalytic
6 March 2018
activity. In this work, NiO is primarily investigated as a possible synergistic catalyst for ORR
Accepted 8 March 2018
on the LSCF surface. The effect of NiO particles on the effective oxygen chemical surface
Available online xxx
exchange coefficient is revealed with the electrical conductivity relaxation (ECR) technique. At 800
C, the coefficient is increased from 3.48 105 to 6.65 105 cm s1 and
4
cm s1 when NiO particles are deposited using the sputter and drop coating
Keywords:
6.9 10
Solid oxide fuel cell
methods, respectively. Adding 5 wt % NiO to LSCF reduces the area specific interfacial
Lanthanum strontium cobalt ferrite
polarization resistance, for example from 0.108 to 0.082 U cm2 at 700 C, as demonstrated
Oxygen reduction reaction
by impedance spectroscopy on symmetric cells using samaria-doped ceria as the electro-
Interfacial polarization resistance
lyte. Adding NiO can also improve the performance of anode supported button cells,
NiO
increasing the peak power density from 0.731 to 1.031 W cm2 at 800 C. On the whole, the increased oxygen surface exchange rate together with the reduced electrode resistance and improved power density, exhibit that NiO is a potential additive to enhance the LSCF catalytic activity. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Efficient, clean and direct conversion of chemical energy into electrical energy can be achieved by the usage of solid oxide fuel cells (SOFC), where oxygen molecules from air are reduced to oxygen ions at the cathode and subsequently
transported to the anode to oxidize the fuel molecules such as H2 and CO. The oxygen reduction reaction (ORR) is believed to be the main contributor towards the electrode polarization resistance. Thus, the highly electrocatalytically active cathodes, especially those which operate at an intermediate temperature range of 600e800 C, have attracted
* Corresponding author. CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering & Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, No. 96 Jinzhai Road, Hefei, Anhui Province, 230026, PR China. E-mail address: [email protected] (C. Xia). https://doi.org/10.1016/j.ijhydene.2018.03.053 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Nadeem M, et al., Effect of NiO addition on oxygen reduction reaction at lanthanum strontium cobalt ferrite cathode for solid oxide fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.053
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much attention over the past years [1,2]. In particular, LaxSr1xCoyFe1yO3-d (LSCF), a member of the perovskite's family, has gained tremendous popularity as the cathode material for intermediate-temperature applications due to its relatively high ionic and electronic conductivity compared with other types of oxide electrocatalysts. Besides being an ORR electrocatalyst, LSCF offers high electrode performance by facilitating a bulk pathway for oxygen anion transport [3e6]. However, despite these benefits, there is hindrance for its practical application as the ORR on the LSCF cathode is limited by the surface exchange process rather than the bulk charge transport process, and it is the cathode reaction which dominantly contributes towards SOFC performance [7e9]. A number of strategies have been adopted in order to enhance the surface reaction kinetics. Initially, noble metals like Pd [10e12], Rh [11], and Ag [13e16] were utilized to enhance the ORR on LSCF because of their high catalytic activity. The LSCF cathode surface was modified by nanoparticles of these metals through the infiltration process. And, the performance had been improved, for example, by the reduction in the area specific interfacial polarization resistance (ASR) at 700 C from 0.14 U cm2 for bare LSCF to 0.044 U cm2 with infiltration of 7.5 mg cm2 Pd, corresponding to an improving factor of 3.2 [12]. However, the noble metals were often avoided due to their high cost. They were replaced with doped ceria materials with the belief that ceria can boost the ORR kinetics by enhancing the surface exchange process as well as increasing the oxygen ionic conductivity. In fact, in comparison with the bare LSCF cathode, Nie et al. [17] did achieve a considerably lower ASR of 0.40 U cm2 at 650 C by infiltrating samaria-doped ceria into the porous LSCF scaffolds. Similarly, Chen et al. [10] demonstrated that upon infiltration of 160 mL of 0.25 mol L1 gadolinia-doped ceria, a polarization resistance of 0.10 U cm2 at 650 C can be obtained. Furthermore, mixed ionic and electronic conductors such as Sm0.5Sr0.5CoO3-d [18], La0.6Sr0.4CoO3-d [19,20], and Pr0.6Sr0.4CoO3-d [21], which are good electrocatalysts for ORR, were also employed to enhance the ORR kinetics. For example, Lou et al. have reported that infiltration of Sm0.5Sr0.5CoO3-d into LSCF cathodes can reduce ASR from 0.103 to 0.036 U cm2 at 750 C, along with an ~22% increment in the peak power densities [18]. In addition to the aforementioned strategies, the ORR on the LSCF cathode has been significantly improved by some rare earth and transition metal compounds, which are neither electronic/ionic conductors nor electrocatalysts. For example, Hong et al. [22] has reported an improving factor of 2.35 at 700 C by infiltrating 8.30 wt % BaCO3. They further demonstrated that coating 0.85 mg cm2 BaCO3 onto the surface of LSCF can increase the kchem of LSCF from 5.2 105 to 75 105 cm s1 at 800 C. Similarly, SrCO3 can also increase the ORR activity by an improving factor of 1.94 at 700 C [23]. Additionally, Zhang et al. [24] have shown that deposition of CaO particles can also increase the kchem. Similarly, the kchem is increased by a factor of about 3 at 750 C with the coating of CuO, as reported by Hong et al. [25]. Recently, Yang et al. reported that the solution deposition of MgO particles on the surface of LSCF can increase the kchem by a factor of up to 2.4 at 750 C [26]. However, Wang et al. [27,28] reported a discrepancy with transition metal compounds and found that
deposition of Cr2O3 on the LSCF surface results in the deterioration of the ORR activity. In this communication, we report the findings on the effect of NiO, a p-type semiconducting material as well as a transition metal compound, on the ORR activity of LSCF. Kinetics of the oxygen reduction reaction were explored with the electrical conductivity relaxation (ECR) technique to determine the NiO effect on kchem. The ORR activity was further evaluated by using symmetric and single cells.
Experimental Powders synthesis La0.6Sr0.4Co0.2Fe0.8O3d (LSCF) perovskite powders were prepared by the citric-EDTA method [29]. Stoichiometric amounts of metal nitrates La(NO3)3.6H2O (99.99%), Sr(NO3)2 (99.5%), Co(NO3)2.6H2O (98.5%) and Fe(NO3)3.9H2O (98.5%) to the nominal composition were dissolved in distilled water to make a solution. Citric acid (99.5%) and EDTA (99.5%) were subsequently added with a molar ratio of citric-acid: EDTA: metalions ¼ 1:1:1. The pH was maintained at ~7.0 with ammonia solution. The LSCF solution was then heated on a hot plate till self-combustion, forming the primary black powders. The assynthesized powders were subsequently calcined at 800 C for 2 h to remove the carbon residue and to obtain fine LSCF perovskites. The glycine-nitrate method was used to prepare Sm0.2Ce0.8O1.9 (SDC) electrolyte [30] and NiO powders, followed by heating the ashes at 600 and 850 C for 2 h, respectively. The NiO and LSCF powders were mixed at the weight ratio of 1:1 and sintered at 1000 C for 10 h to check the chemical compatibility. All the chemicals were purchased from Sinopharm Chemical Reagent Co. Ltd., China.
Cell fabrications Symmetrical cells were composed to evaluate the effect of NiO on the interfacial polarization resistance. Disc-shaped SDC pellets were prepared by uniaxially pressing the SDC powder at 250 MPa using a 13 mm diameter stainless steel die, followed by sintering at 1350 C for 5 h to form dense SDC pellets as the electrolyte substrates. To fabricate LSCF cathode, LSCF slurry was prepared by mixing the LSCF powder with an organic binder (a-terpineol as solvent and ethyl cellulose as the binder). The slurry was then screen-printed onto both sides of the SDC pellets and heated at 1000 C for 2 h to form symmetrical cells. To design composite cathodes, NiOLSCF composite powders with 5 and 10 wt % NiO were prepared by adding appropriate amounts of Ni(NO3)2.6H2O to the LSCF solution, followed by heating and calcination processes, the same as preparing the LSCF powders. The designing process of 5 and 10 wt % NiO-LSCF composite cathodes was also same as for the bare LSCF cathode. The electrochemical performance was further evaluated using anode-supported single cells with yttria-stabilized zirconia (YSZ), obtained from Sinopharm Chemical Reagent Co. Ltd., China, as the electrolyte, Ni-YSZ as the anode and SDC as the interlayer between YSZ and LSCF. Firstly, NiO-YSZ anode-support was tap casted and pre-fired at 900 C for 2 h. Then, an active
Please cite this article in press as: Nadeem M, et al., Effect of NiO addition on oxygen reduction reaction at lanthanum strontium cobalt ferrite cathode for solid oxide fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.053
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NiO-YSZ layer and YSZ electrolyte were deposited onto the anode support by a particle suspension coating process followed by co-firing at 1400 C for 5 h. To prevent the reaction between LSCF cathodes and YSZ electrolyte, SDC buffer layer was then applied on the YSZ surface and fired at 1200 C for 2 h. The cathodes were fabricated in the same manner as the symmetrical cells.
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was obtained by heating the mixture of LSCF and NiO in the weight ratio of 1:1 at 1000 C for 10 h. Fig. 1a indicates a well fitted single phase perovskite structure of LSCF with cell parameters a ¼ b ¼ 0.5475 nm, c ¼ 1.347 nm, a ¼ b ¼ 90 and
Electrical conductivity relaxation measurement The electrical conductivity relaxation (ECR) technique was used to demonstrate the effect of NiO particles on the surface oxygen reduction kinetics. The dense rectangular bars were prepared by pressing the LSCF powders at 300 MPa and sintering at 1400 C for 5 h in air [22]. The sintered bars, with size dimensions 32 5.48 0.60 mm3, had relative density greater than 97% of the theoretical value measured by the Archimedes method. Two methods were adopted to grow NiO particles on the bar surfaces. In the first method, the 0.05 ml, 0.10 ml, 0.15 ml and 0.20 ml Ni(NO3)2.6H2O solutions, with the concentration of 0.3 mol L1, were drop coated onto the bar surfaces, half volume per each of the two bar sides. This was followed by heating the bars at 800 C to form the NiO particles on the bar surfaces. In the second method, bar surfaces were coated with nickel films using sputter deposition (JFC-1600, JEOL). The coating was carried out for 60 s on both sides of the bar at 20 mA under a pressure of 10 Pa in air. This was followed by heating the Ni film in nitrogen atmosphere at 800 C for 2 h to form Ni particles, and then subsequently in air at 800 C for 2 h to form NiO particles. The conductivity was measured using the four-probe method with a digital multimeter (Keithley, 2001-785D). The gas atmosphere was abruptly changed from oxygen partial pressure PO2 ¼ 0.21 atm (O2 þ N2, Nanjing special gas Factory Co., Ltd.) to PO2 ¼ 1 atm (O2). The total gas flow rate was maintained at 200 ml min1 to ensure that the PO2 of the gas achieved equilibrium in no more than 20 s.
Sample characterization The XRD patterns were obtained using the technique of X-ray diffraction (XRD, Rigaku TTR-III diffractometer) with a Cu-Ka radiation source in the range of 20e80 . The morphologies were characterized using scanning electron microscopy (SEM, JSM-6700F). Electrochemical measurements were conducted with a Zahner Im6e electrochemical workstation. The impedance of the symmetric cells was measured in ambient air, and the frequency ranged from 101e106 Hz with AC signal amplitude of 10 mV. The performance of the single cell was evaluated using humidified (~3% H2O) hydrogen as the fuel, with a flow rate of 25 ml min1, and ambient air as the oxidant. All impedance spectra for single cells were recorded under open-circuit conditions with 10 mV AC signal perturbation.
Results and discussion Compatibility between LSCF and NiO Fig. 1aec shows the rietveld refinement analysis of LSCF, NiO and LSCF-NiO composite, respectively. The composite powder
Fig. 1 e The Rietveld refinement analysis of (a) LSCF powder, (b) NiO powder, and (c) LSCF-NiO composite powders heated at 1000 C for 10 h.
Please cite this article in press as: Nadeem M, et al., Effect of NiO addition on oxygen reduction reaction at lanthanum strontium cobalt ferrite cathode for solid oxide fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.053
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g ¼ 120 . In LSCF-NiO composite powder (Fig. 1c), well fitted peaks are distinguishable and can be assigned to either LSCF or NiO. The cell parameters of LSCF in the composite powder are a ¼ b ¼ 0.5471 nm, c ¼ 1.349 nm, a ¼ b ¼ 90 and g ¼ 120 . No impurity peak was observed. The peak positions and cell parameters do not change in rietveld refinement analysis of composite powder, which confirms the good compatibility between LSCF and NiO under the testing conditions.
NiO effects on the surface exchange rate The NiO effect on the oxygen surface reduction rate was investigated using LSCF bars coated with various amounts of NiO particles via solution deposition method. Fig. 2 displays a surface overview of bare LSCF and NiO coated LSCF samples. Fig. 2a displays a dense LSCF surface, the grain is up to 10 mm in size and the grain-boundaries are clearly presented. The relative density of LSCF bars was found to be more than 97%, as determined by the Archimedes method in distilled water. In Fig. 2b, fine NiO particles can be seen covering the LSCF surface. The particles must be NiO since they were obtained by heating the drop-coated Ni(NO3)2.6H2O solution. The fine particles are not so uniform and naked LSCF surface is observable at some places due to the low coating amount of NiO particles. In addition, the LSCF grain boundaries are observable. As the coating amount is increased, the NiO
particles form a porous monolayer in a more uniform manner (Fig. 2c and d, respectively). The greater coating amount increases the thickness of the monolayer such that some cracks develop in the surface as shown in Fig. 2e. NiO decoration effect on the oxygen surface exchange kinetics of LSCF was explored with the electrical conductivity relaxation technique. Fig. 3a shows normalized conductivity profiles and their fitting curves for bare and NiO decorated LSCF bars as a function of relaxation time at 800 C, with a PO2 step change from 0.21 to 1 atm oxygen. A good agreement was observed between experimental data and fitting curves. The bare LSCF sample takes about 6200 s to attain equilibrium. When LSCF is drop coated with 0.05 ml Ni(NO3)2.6H2O solution, the relaxation time is reduced to 900 s. In our study, the bulk transport properties are the same and the driving force conditions are also identical. Therefore, the reduction in the relaxation time can be purely attributed to the enhancement in oxygen surface exchange process associated with NiO particles. The LSCF samples display 600 and 470 s relaxation times for drop coating of 0.10 ml and 0.15 ml Ni(NO3)2.6H2O solution, respectively. However, the further drop coating of 0.20 ml Ni(NO3)2.6H2O solution leads to a relative increase in the relaxation time from 470 to 525 s. The 0.15 ml drop coating is thus optimal, when the oxygen surface exchange process occurs at its maximum rate. The relaxation time of 0.15 ml drop coated LSCF is 13% less in magnitude than the bare LSCF,
Fig. 2 e Surface SEM micrographs for (a) a bare LSCF bar and LSCF bars drop coated with Ni(NO3)2.6H2O solution of volume (b) 0.05 ml, (c) 0.10 ml, (d) 0.15 ml, and (e) 0.20 ml. Please cite this article in press as: Nadeem M, et al., Effect of NiO addition on oxygen reduction reaction at lanthanum strontium cobalt ferrite cathode for solid oxide fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.053
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surface in a discontinuous manner as shown in Fig. 4a. The NiO particles are grown on the LSCF surface by the sputter deposition method for 60 s. Compared to the bare LSCF, the relaxation time drops to 3400 s and kchem increases to 6.65 105 cm s1 at 800 C suggesting an improvement in the oxygen surface exchange process. Fig. 4b compares kchem for different samples, demonstrating that the surface exchange rate can be improved by both drop coated and sputter deposited NiO particles by a factor of ~20 and ~2, respectively. Nevertheless, in the case of bare LSCF, the oxygen surface exchange reaction takes place only on the surface of the bare LSCF i.e. at the LSCF-gas two phase interface. After NiO decoration, the oxygen exchange reaction can proceed not only at the LSCF-gas interfaces, but also at the LSCF-NiO-gas three-phase boundary. NiO decoration reduces the exposed surface area of LSCF to gas phase, while increases the length of the three-phase boundaries. Therefore, it is possible that the ORR can proceed more easily at the LSCF-NiO-gas threephase boundaries, hence the improved oxygen surface exchange kinetics.
Electrochemical performance of LSCF-NiO composite electrode The effect of NiO on the ORR of LSCF was also investigated with impedance spectroscopy. Fig. 5 shows ASR in the temperature range of 550e800 C for symmetrical cells composed of SDC electrolyte supported LSCF and LSCF-NiO composite
Fig. 3 e (a) Normalized conductivity data and fitting curves, and (b) chemical oxygen surface exchange coefficients (kchem) for LSCF drop coated with Ni(NO3)2.6H2O solution. suggesting a significant improvement in oxygen surface exchange rate. The chemical surface exchange coefficients, kchem (cm s1), were obtained by non-linear least squares fitting of the normalized conductivity profiles shown in Fig. 3a. At 800 C, the surface exchange coefficient of the bare LSCF is calculated to be 3.48 105 cm s1, which agrees well with the literature values in the range of 2.4e3.6 105 cm s1 at the same temperature [31e33]. Fig. 3b compares the kchem values at different temperatures. The increase in the drop coating volume of Ni(NO3)2.6H2O solution from 0.05 to 0.15 ml, increases kchem at 800 C from 2.97 104 to 6.9 104 cm s1, well above 3.48 105 cm s1 for the bare LSCF. The effective surface exchange coefficient is improved by a factor of ~20. Further drop coating of 0.20 ml volume, reduces kchem to 6.6 104 cm s1. Thus, 0.15 ml drop coating of Ni(NO3)2.6H2O solution is optimal for the maximum increase in kchem. The NiO effect on kchem was further investigated with LSCF bar deposited with isolated particles. NiO nanoparticles of about 300 nm in size are uniformly distributed over the LSCF
Fig. 4 e (a) Surface SEM micrograph for LSCF bar decorated with NiO for 60 s, and (b) chemical oxygen surface exchange coefficients (kchem) for sputter and drop coated LSCF bars.
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Fig. 5 e Electrode area specific resistances (ASR) for LSCF and LSCF-NiO composite electrodes.
electrodes. The ASR for the bare LSCF electrode is 0.108 U cm2 at 700 C, which is lower than the previously reported resistance of 0.16 U cm2 at the same temperature [23]. The different fabrication procedures including powder synthesis, electrolyte sintering temperature and electrode heating conditions may cause the difference in microstructure and consequently the resistance. The incorporation of 5 wt % NiO into the LSCF, results in the reduction of the ASR from 0.108 to 0.0802 U cm2 at 700 C, corresponding to an improving factor of 1.35, justifying the enhancing properties of NiO towards the activity of the LSCF electrocatalyst. In contrast, 10 wt % NiO addition leads to the higher ASR of 0.157 U cm2 at 700 C, compared to the bare LSCF electrode. Shown in Fig. 6a are the impedance spectra for bare LSCF and LSCF-NiO composite electrodes measured at 700 C in air. The resistances of the electrolyte and lead wires were subtracted for the sake of better understanding the electrode resistances. Although the ORR at cathodes has discrepancies, however, it is generally considered that the reaction contains many elementary processes including oxygen gas diffusion, oxygen surface adsorption/dissociation and incorporation,
Fig. 6 e (a) Electrochemical impedance spectra at 700 C for symmetrical cells with bare LSCF and LSCF-NiO electrodes, (b) the equivalent circuit for fitting the impedance spectra, and (c) the high and low frequency resistance versus NiO content. Please cite this article in press as: Nadeem M, et al., Effect of NiO addition on oxygen reduction reaction at lanthanum strontium cobalt ferrite cathode for solid oxide fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.053
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and oxygen ion migration at the electrode-electrolyte interface [34e36]. The LSCF electrode reveals two depressed arcs in the impedance spectrum, a high frequency arc and a low frequency arc. ASR is obtained by calculating the difference in the high and low-frequency intercepts at the real axes. After NiO addition, the spectra show the same characteristics as bare LSCF, implying that there are two rate-limiting steps. To understand these steps, equivalent circuit of L1R0(R1/CPE1)(R2/ CPE2) was used to fit the spectra using Zview program, Fig. 6b [34,37]. In the circuit, L1 is the inductance connected in series with a resistive element R0, representing the resistance of
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electrolyte and lead wires, which is not shown in the impedance spectra. The circuit elements R1 and R2, parallel with a constant phase element (CPE), are resistances according to high and low-frequency arcs, respectively. Fig. 6c represents the variation in high and low frequency resistance upon NiO addition to LSCF electrode. The high frequency resistance R1 remains almost same suggesting that the process associated with R1 does not change with NiO addition. On the other hand, low frequency resistance R2 shows much change. For example, it decreases from 0.09 to 0.06 U cm2 at 700 C with 5 wt % addition of NiO. Maier et al. [38,39] and Liu et al. [8] have
Fig. 7 e (a) SEM crosssectional view of a single cell with four-layer structure using bare LSCF cathode, (b) enlarged view of the porous LSCF cathode, (c) cell voltage and power density as a function of current density measured at 800 C for single cells using LSCF, LSCF-NiO, LSCF-SDC, and LSCF-SDC-NiO cathodes, and (d) impedance spectra measured under open circuit conditions. Please cite this article in press as: Nadeem M, et al., Effect of NiO addition on oxygen reduction reaction at lanthanum strontium cobalt ferrite cathode for solid oxide fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.053
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correlated R1 with oxygen ion transfer across the electrodeelectrolyte interface, while R2 with oxygen surface exchange reaction. The reduction in R2 suggests that NiO has significant synergistic ORR activity and accelerates the oxygen incorporation process, agreeing well with the ECR results.
Single cell performance The electrochemical performance was further evaluated with anode-supported single cells. Fig. 7a represents the cross sectional view of an anode-supported single cell comprising Ni-YSZ anode, YSZ electrolyte of ~13 mm thickness, SDC buffer layer of ~3 mm thickness and porous LSCF cathode. Fig. 7c compares the current-voltage curves and the corresponding power densities when humidified hydrogen was used as the fuel and ambient air as the oxidant. The cell with single phase LSCF cathode shows a peak power density of 0.731 W cm2 at 800 C, which is comparable with previous experimental data [40]. The peak power density increases to 1.03 W cm2 when LSCF-NiO composite with 5 wt % NiO is used as the cathode. The positive effect of NiO on the electrochemical performance was also observed with LSCF-SDC composite cathodes. The cell with LSCF-SDC composite cathode performs with a higher peak power density of 0.837 W cm2, which is further increased to 0.858 W cm2 with the LSCF-SDC-NiO cathode. Fig. 7d displays impedance spectra measured at 800 C under open circuit conditions. The cell with the LSCF-NiO composite cathode gives the total interfacial polarization resistance of 0.124 U cm2, lower than the resistance of 0.222 U cm2 for the bare LSCF cathode. Similarly, the resistance for the cell with LSCF-SDC-NiO composite cathode is reduced to 0.144 from 0.164 U cm2 for the LSCF-SDC cathode based cell. The increment in peak power densities and reduction in total interfacial polarization resistances must derive from the enhanced electrochemical performance associated with NiO since the anode and electrolyte nature is same in all the cells.
Conclusion NiO has been investigated as the synergistic catalyst for the oxygen reduction reaction on LSCF. The decoration of NiO particles by the drop coating and sputter deposition methods result in the successful acceleration of the oxygen surface exchange rate, increasing the effective chemical oxygen surface exchange coefficient by a factor of ~20 and ~2, respectively. Adding 5 wt % NiO to LSCF can reduce the interfacial polarization resistance of symmetrical cells from 0.108 to 0.082 U cm2 at 700 C and increase the peak power density of single cells from 0.731 to 1.031 W cm2 at 800 C. Thus, NiO is a potential material which can be used synergistically to enhance the kinetics of the oxygen reduction reaction on the LSCF cathode.
Acknowledgements We are very grateful to the financial support from National Natural Science Foundation of China (91645101).
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Please cite this article in press as: Nadeem M, et al., Effect of NiO addition on oxygen reduction reaction at lanthanum strontium cobalt ferrite cathode for solid oxide fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.03.053