Performance and stability of Ruddlesden-Popper La2NiO4+δ oxygen electrodes under solid oxide electrolysis cell operation conditions

Author’s Accepted Manuscript Performance and stability of Ruddlesden-Popper La2NiO4+δ oxygen electrodes under solid oxide electrolysis cell operation conditions Xin Tong, Feng Zhou, Shengbing Yang, Shaohua Zhong, Mingrui Wei, Yihui Liu www.elsevier.com/locate/ceri

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S0272-8842(17)30931-8 http://dx.doi.org/10.1016/j.ceramint.2017.05.130 CERI15290

To appear in: Ceramics International Received date: 27 April 2017 Revised date: 16 May 2017 Accepted date: 17 May 2017 Cite this article as: Xin Tong, Feng Zhou, Shengbing Yang, Shaohua Zhong, Mingrui Wei and Yihui Liu, Performance and stability of Ruddlesden-Popper La2NiO4+δ oxygen electrodes under solid oxide electrolysis cell operation c o n d i t i o n s , Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.05.130 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Performance and stability of Ruddlesden-Popper La2NiO4+δ oxygen electrodes under solid oxide electrolysis cell operation conditions Xin Tonga,b,c, Feng Zhoua, Shengbing Yanga,c*, Shaohua Zhonga, Mingrui Weia, Yihui Liua,b* a

Hubei Key Laboratory of Advanced Technology for Automotive Components & Hubei

Collaborative Innovation Center for Automotive Components Technology（Wuhan University of Technology）, Wuhan 430070, China b

Center for Fuel Cell Innovation, School of Materials Science and Engineering, State Key

Laboratory of Material Processing and Die &Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, China c

School of Automotive Engineering, Wuhan University of Technology, Wuhan 430070, China [email protected] [email protected]

*

Corresponding authors.

Abstract Ruddlesden-Poper La2NiO4+δ (LNO) oxygen electrodes were investigated under solid oxide electrolysis cell (SOEC) operation conditions. The electrochemical performance of LNO was measured in both solid oxide fuel cell (SOFC) and SOEC modes at 750 C in air. The results suggest that LNO oxygen electrodes exhibit high electrochemical activity and the processes related to oxygen adsorption, dissociation and diffusion dominate the oxygen evolution reaction on the electrodes. Electrical conductivity relaxation (ECR) measurements imply that LNO shows better oxygen surface exchange performance than conventional LSM and LSCF electrodes, because of its special crystal structure with flexible non-stoichiometric 1

oxygen. Significant performance degradation was observed during polarization at 500 mA cm-2 and 750 C in the SOEC mode for 48 h. XRD and XPS results confirmed that high-order Ruddlesden-Popper La3Ni2O7 and La4Ni3O10 phases have great contributions to the performance degradation of LNO oxygen electrodes related to anodic current polarization at 500 mA cm-2 and 750 C. Keywords: SOEC; Ruddlesden-Popper; La2NiO4+δ; ECR; Degradation.

1. Introduction Solid oxide electrolysis cell (SOEC), which converts the electric energy and heat energy into chemical energy, is an efficient technology for hydrogen production and energy storage in the form of fuels like hydrogen[1, 2]. The high operation temperature of SOEC can effectively reduce the power demand in the electrolysis process and accelerate electrode reaction process, thus reducing the cost of hydrogen production and improving electrolysis efficiency[3]. Combining SOEC with nuclear power station or renewable energies is a promising technology to combat the energy crisis and environmental problems[4]. In principle, SOEC is operated in the inverse mode of solid oxide fuel cell (SOFC). Although benefiting from the high operation temperature, SOEC still needs to lower the operation temperature in consideration of the material sustainability and the durability of the device[5]. Therefore, developing mixed ionic and electronic conductors (MIECs) as oxygen electrodes of SOEC is a promising solution. Ruddlesden-Popper Ln2NiO4+δ （Ln=La, Pr, Nd） oxides are a kind of MIECs with special crystal structure, in which LnNiO3 perovskite structure layer and LnO salt layer are 2

alternately stacked along the C axis. In order to maintain the structural stability, there is non-stoichiometric oxygen inserted into LnO salt layer and localized holes are generated around the Ni ions. At high temperature, the high ion mobility of the non-stoichiometric oxygen is responsible for high ionic conductivity while holes contribute to electronic conduction[6]. Ln2NiO4+δ also show better oxygen surface exchange performance and bulk transfer performance than conventional perovskite-type LaxSr1-xCoyFe1-yO3-δ (LSCF) and LaxSr1-xNiyFe1-yO3-δ (LSNF) oxides due to flexible non-stoichiometric oxygen[7]. Ln2NiO4+δ oxides have received much attention as SOFC cathodes. The research results of Montenegro-Herna´ndez et al. indicate that LNO is thermal stable and chemically compatible with GDC and YSZ during annealing at 600 - 800 C[8]. Y. Lee et al. reported that LNO exhibited excellent performance as an SOFC cathode at 650 -800 C[9]. Similar results were also obtained by Hildenbrand et al.’ research[10]. Previous studies show that many SOFC cathode materials can be directly operated in SOEC mode[11-13]. MIECs, such as LSCF[14], BaxSr1-xCoyFe1-yO3-δ (BSCF)[15] are commonly used as SOFC cathodes because they can improve the catalytic activity of electrodes due to the increased length of triple phase boundary[16]. These electrodes have also been reported to operate in SOEC mode and show excellent performance[17, 18]. Similarly, Ln2NiO4+δ can be applied as oxygen electrodes of SOEC[19-21]. Kim et al. compared the polarization and stability performance of LNO with LSCF as SOEC electrodes, and concluded that LNO showed better electrochemical performance than that of LSCF[22]. Y.S. Yoo et al. also revealed that LNO oxygen electrodes had stable performance in SOEC mode at 600 C[23]. However, the reaction mechanisms of LNO oxygen electrodes of SOEC are still not clear and should be investigated in detail. 3

The present study focuses on the electrochemical activity, polarization characteristic and performance degradation of LNO oxygen electrodes under SOEC operation conditions. Electrochemical measurements were carried out to evaluate the polarization behaviors and the stability of LNO oxygen electrodes. The oxygen surface exchange coefficients of LNO were measured by electrical conductivity relaxation (ECR) method. Degradation mechanisms were discussed by characterizing the phase composition and the surface chemistry of LNO electrodes before and after anodic current polarization at 500 mA cm-2 at 750 C.

2. Experimental 2.1 Preparation of samples LNO powders were synthesised by citrate-nitrate method. Stoichiometric amounts of La(NO3)3·6H2O, Ni(NO3)2·6H2O (Sinopharm Chemical Reagent Co. Ltd.) were dissolved in distilled water and mixed well. Citric acid was then added to the solution with a molar ratio of citric acid to metal ion of 1.1:1. The solution was heated to 80 C and stirred until the green gel was obtained. The gel was dried at 120 C for 5 h, and then sintered at 1000 C for 5 h to obtain pure LNO powders. Dense Gd0.1Ce0.9O2-δ (GDC) electrolyte pellets were prepared from GDC powders (Ningbo SOFCMAN Energy Technology Co. Ltd, China) by die pressing and sintering at 1550 C for 6 h in air. LNO ink was obtained by mixing LNO powders and cellulose binder in a weight ratio of 5:5, followed by grinding. The ink was then painted on dense GDC electrolyte substrates as the working electrode. After sintering at 1000 C in air for 2 h, porous electrodes were obtained. In order to investigate oxygen surface exchange property by ECR measurements, 4

rectangular dense LNO bar samples were prepared by dry pressing method and sintered at 1350 C in air for 5 h. 2.2 Characterization measurements The three-electrode configuration was used for electrochemical measurements. Pt paste was applied on the GDC electrolytes and sintered at 850 C in air for 2 h, to form counter and reference electrodes. Counter electrodes were located in the center of GDC electrolyte opposite to LNO working electrodes. Reference electrodes were annular around the counter electrodes. Electrochemical impedance spectroscopy (EIS) of LNO electrodes was measured by Gamry Interface 1000 in a frequency range between 0.1 HZ and 100 KHZ with signal amplitude of 10 mV. The electrochemical impedance responses under open circuit in air were measured from 650 to 800 C. IR free overpotential curves in both SOFC and SOEC modes were measured at 750 C in air. The stability of LNO electrodes under SOEC operation conditions was characterized by electrochemical impedance responses after anodic current polarization at 500 mA cm-2 and 750 C in air for 48 h. The electrical conductivity of LNO was measured by DC four-probe method using a measurement system including a digital multimeter (Keithley Model 2000) and a program written by the LABVIEW software. Four silver threads wrapped around the ends of strip dense samples, which were attached to the bar surface using Ag paste followed by calcination at 800 C in air for 2 h. Oxygen surface exchange coefficient of LNO was measured by ECR method. During the test, the total gas flow rate is 100 ml min-1. The oxygen partial pressure changes from 0.21 atm (21%O2 + 79%N2) to 1 atm (pure O2). The electrical conductivity was recorded with time until a new equilibrium was reached. 5

The phase composition of LNO powders and LNO electrodes before and after polarization was characterized by X-ray diffraction (XRD) (XRD-7000, Shimadzu) with Cu Kα radiation. GSAS-EXPGUI software was applied to perform Rietveld refinement. The Chebyshev polynomial and Pseudo-Voigt function was adopted to model the background intensities and peaks profiles, respectively. The least-square optimization was converged to minimize the profile parameter (Rp), weighted profile parameter (Rwp), reduced chi-square (χ2) with acceptable structural parameters. Surface chemistry of LNO electrodes before and after polarization was characterized by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). Spectrometer calibration was done with reference to C1s spectra, which were assumed to be fixed at 284.6 eV.

3. Results and discussion 3.1 Phase composition Refined XRD patterns are shown in Fig.1. The unit cell parameters calculated by XRD refinements are listed in Table 1.The model that presents the best fit to the experimental data at room temperature is the Fmmm orthorhombic. The Rp, Rwp, χ2 are 2.05%, 2.65%, 1.92 respectively, showing the refinement results are reasonable. The refinement results confirm that LNO powders sintered at 1000 C are single phase with Ruddlesden-Popper structure and the space group is Fmmm-type at room temperature. Similar results have been obtained by in-situ high temperature neutron powder diffraction, showing that LNO is Fmmm-type structure below 150 C[24]. Non-stoichiometric oxygen coefficient δ of LNO powders was measured by the iodometric titration, which used the reduction of iodine to achieve the measurement of Ni3+ content. From the test results, δ of the powders is 0.16, similar to that of thermogravimetric analysis obtained by E. Boehm et al.[25]. Oxygen ionic conductivity of 6

LNO is predominantly via migration of non-stoichiometric oxygen[26], this indicates that as-prepared LNO has good ionic conductivity, is potential to work as SOEC oxygen electrodes. 3.2 Electrochemical characterization Fig. 2 shows the initial IR-free overpotential curves in SOEC and SOFC mode for LNO electrodes at 750 C in air. In SOFC mode, the polarization behavior of LNO electrodes is almost linear. The overpotential of LNO oxygen electrode is 61 mV at 200 mA cm-2, which is lower than that of LSCF+Ag composite electrodes (~80 mV) [27]. In SOEC mode, the polarization behavior in low current density range is also linear while the overpotential tends to flatten in high current density range. T. Ogier et al. have observed similar phenomenon in Pr2NiO4+δ oxygen electrodes and deduced that the sum of overpotentials mainly originated from the activation overpotential of charge transfer processes[19]. The overpotential of LNO oxygen electrodes is 107 mV at 500 mA cm-2 and 750 C, which is lower than that of LSM (380 mV at 800 C)[28] and even comparable to that of LSCF (0.08 V at 900 C)[29]. The low overpotentials imply that LNO exhibits outstanding electrocatalytic activity toward both oxygen reduction and evolution reactions. Fig.3 is the impedance response under open circuit for the oxygen evolution reaction on LNO oxygen electrodes. It can be found that the polarization resistance (Rp) is 3.86, 1.84, 0.39, 0.14, 0.049 Ω cm2 at 600, 650, 700, 750, 800 C, respectively. In previous research, Rp for pure LSM electrode is 5.6 Ω cm2 at 800 C[30], while Rp for pure Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) electrode is 0.66 Ω cm2 at 750 C[18]. This indicates that the electrochemical activity for oxygen evolution reactions of LNO electrodes is better than that of LSM and BSCF 7

electrodes. Furthermore, the impedance response at 600 C was distinctly separated into two arcs (Fig. 3a), implying there were two reaction processes on LNO electrodes. Fig. 4a shows RP and activation energy (Ea) of LNO electrodes for the oxygen evolution reaction. The activation energy is 169.76 KJ mol-1 at 600-800 C, which is close to the activation energy value of 154.56 KJ mol-1 (1.61 eV) for the surface exchange process, reported in previous literature[31]. This indicates that the oxygen evolution reaction on LNO electrodes may be affected by the oxygen surface exchange process. In order to understand the effects of different reaction processes on electrode performance, EIS was fitted with an equivalent circuit as shown in Fig. 3f. In this equivalent circuit, RΩ corresponds to ohmic resistance, mainly caused by electrolyte; R and Q are polarization resistance and constant phase element, respectively. Based on previous literatures[32, 33], the impedance responses of LNO oxygen electrodes in high frequency are related to processes of oxygen ions migration and diffusion between GDC electrolytes and LNO oxygen electrodes, and these in low frequency are related to the surface diffusion and combination of oxygen species for the oxygen evolution reactions. EIS fitted by the equivalent circuit is shown in Fig. 3a, b, c, d, e. Fig. 4b shows Arrhenius plots of the polarization resistance in high frequency (RH) and low frequency (RL) of LNO electrodes at different temperatures. Ea for RH and RL of LNO electrodes is 80.25, 178.19 KJ mol-1, respectively. Therefore, reactions on electrodes are mainly dominated by the low frequency process, which is related to the surface diffusion and combination of the oxygen species. Ea for RH of pure LSM oxygen electrodes is reported as ~100 KJ mol-1[34],which is higher than that of LNO oxygen electrodes. This can be explained that the oxygen migration at LNO/electrolyte interface is faster than that at 8

LSM/electrolyte interface because LNO shows better ion conductivity than LSM.

However,

Ea for RL of LNO oxygen electrodes is 178.19 KJ mol-1, significantly higher than that on LSM-YSZ composite oxygen electrodes (125 KJ mol-1)[35]. The higher activation energy may be due to high structural stability, which needs extra oxygen in LaO salt layer in LNO crystal lattice [36]. Therefore, operation temperature has a great influence in the low frequency region. 3.3 Oxygen surface exchange property ECR measurements were applied to characterize oxygen surface exchange performance of the electrodes. Fig 5 shows the normalized conductivities of LNO and LSCF samples at 800 C when the oxygen partial pressure varied suddenly from 0.21 to 1 atm. The experimental data was fitted according to the equations mentioned by Y. L. Huang[37]. The surface exchange coefficient K of LNO and LSCF is 2.5×10-4 and 8×10-5 cm s-1, respectively, suggesting that LNO shows better oxygen surface exchange performance than LSCF. LNO also shows better oxygen surface exchange performance than perovskite LSM oxides (K= 9×10-5 cm s-1 at 1000 C)[38]. For perovskite-type oxides, the oxygen surface exchange process can be described as following reaction: 1 2

∙ O2(g) + VO′′ ↔ O× O +2h

(1)

∙ O2(g) , VO′′ , O× O , h represent gas phase oxygen, oxygen vacancies, oxygen ion and electron

holes, respectively. The surface exchange process is controlled by both electron concentrations and oxygen vacancy concentrations[38]. There are more oxygen exchange sites in Ruddlesden-Popper LNO oxides because LaO salt layer contains O2- forming interstitial oxygen besides oxygen vacancy in perovskite layer[7]. Meanwhile, O2- diffusion 9

via interstitial oxygen is greater than that via oxygen vacancies[7, 26], which accelerates the oxygen surface exchange process. Therefore, these Ruddlesden-Popper LNO oxides have excellent oxygen surface exchange performance. Generally, surface exchange processes are often the limit step of oxygen reactions[39]. LNO has fast electrode reaction rate because of excellent oxygen surface performance, which is a very attractive advantage compared with LSM and LSCF materials. 3.4 Performance stability In order to test performance stability of LNO oxygen electrodes, the cell was operated under a constant anodic current of 500 mA cm-2 at 750 C in air for 48 h. Fig. 6a is the electrochemical impedance spectra of LNO oxygen electrodes as a function of time. The ohmic resistance (RΩ) and Rp are obtained by the spectra and detailed results are shown in Fig. 6b. It can be found that the value of RΩ before and after the polarization for 48 h is 1.108 and 1.081 Ω cm-2, respectively. RΩ changes little, indicating no delimination occurs at the interface between LNO oxygen electrodes and GDC electrolytes. The variety of Rp as a function of anodic polarization time can be divided into two stages: Ⅰ rapidly increasing stage (0 ~ 8 h), Ⅱ stable stage (8 ~ 48 h) . In stageⅠ, Rp changes to 0.154 from 0.334 Ω cm-2 and the deterioration rate even exceeds 100%. But when polarization time is prolonged to stageⅡ, Rp tends to constant. In order to understand degradation mechanisms in the initial stage of anodic current polarization, the phase composition of the LNO electrodes before and after anodic polarization at 500 mA cm-2 for 8 h was measured by XRD, and results are shown in Fig. 7. Only peaks of LNO and GDC are found in the patterns of sample before polarization. After

10

anodic polarization for 8 h, peaks of high-order Ruddlesden-Popper La3Ni2O7, La4Ni3O10 phases and La2O3 are observed. Prolonging polarization time to 12 h, La3Ni2O7, La4Ni3O10 and La2O3 phases are also identified in XRD results. However, R.K. Sharma el al. reported no decomposition of LNO films on GDC electrolyte after 30 days in air at 800 C[40]. This indicates that anodic current polarization causes the decomposition of LNO electrodes. The stability of LNO oxygen electrodes in SOEC mode should be improved and element doping may be an effective solution[41, 42]. XPS was used to investigate the surface chemistry of LNO electrodes before and after polarization treatment. Fig. 8 shows the XPS spectra of O 1s in LNO electrodes before and after anodic current polarization at 500 mA cm-2 for 8 h. Binding energies and relative atomic concentrations of different oxygen states are summarized in Table 2. Binding energies for O 1s peaks can be divided into four peaks. The peaks with higher binding energy at 532.96 and 531.47 eV are related to adsorbed water and organic compounds, respectively[43, 44]. Lower binding energy peaks at 529.81 and 528.85 eV are related to the typical of metal-oxygen bonds and can be attributed to lattice oxygen[45, 46]. La-O bond has a strong ionic character and Ni-O bond has a covalent one[47]. Binding energies at 529.81, 529.33 and 529.50 eV correspond to the oxygen in perovskite layer with Ni-O bond, while binding energies at 528.85, 528.19 and 528.55 eV are attributed to the oxygen of La-O bond in rock-salt layer tied with perovskite layer[48]. Oxygen concentration in perovskite layer is 20.48% in as-prepared samples and increases to 43.66%, while oxygen concentration in rock-salt layer decreases from 20.69% to 10.05% after polarization for 8 h. It can be concluded that polarization treatment increases oxygen contents in perovskite layer, indicating the formation 11

of La3Ni2O7, La4Ni3O10 and La2O3 phases. This is consistent with XRD results (Fig. 7). After polarization for 12 h, oxygen concentration in perovskite layer and rock-salt layer is 44.65 and 11.33%, respectively, which is similar to that of electrodes after the polarization for 8 h. Therefore, the formation of high-order Ruddlesden-Popper La3Ni2O7, La4Ni3O10 phases is completed after polarization for 12 h. The research of Sharma et al. shows that Rp of La3Ni2O7 and La4Ni3O10 electrodes with GDC electrolytes is 1.0 and 1.5 Ω cm2 at 700 C, respectively[49]. Lou et al.’s research shows that Rp of La3Ni2O7 electrodes is 0.39 Ω cm2 at 750 C[50]. These values are significantly higher than that of LNO electrodes (Rp = 0.14 Ω cm2 at 750 C) in this study. It can be concluded that the catalytic activities of La3Ni2O7 and La4Ni3O10 oxides are worse than LNO. Therefore, combination with the variation of Rp shown in Fig. 6, the formation of La3Ni2O7 and La4Ni3O10 oxides related to anodic current polarization has great contributions to performance degradation of LNO electrodes.

4. Conclusions The performance and stability of LNO oxygen electrodes of SOEC were investigated under anodic current polarization at 500 mA cm-2 and 750C. The results of electrochemical and ECR measurements prove that LNO has excellent catalytic activities because of its excellent oxygen surface exchange performance. Stability testing of LNO oxygen electrodes shows that significant degradation occurs in the initial stage of anodic polarization at 500 mA cm-2. XRD and XPS results show that anodic polarization results in the decomposition of LNO electrodes and the formation of high-order Ruddlesden-Popper La3Ni2O7 and La4Ni3O10 oxides, which is considered to have great contributions to performance degradation of LNO electrodes under anodic polarization at 500 mA cm-2. 12

Acknowledgements The project was supported by State Key Laboratory of Materials Processing and Die &Mould Technology, Huazhong University of Science and Technology (P2015-06), the National Natural Science Foundation of China (51402227). XRD and SEM examinations were assisted by the Center of Material Research and Analysis of Wuhan University of Technology.

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Fig. 1 Refined XRD patterns of as-prepared LNO powders.

16

Fig. 2 IR-free overpotential curves versus current density in SOEC and SOFC modes at 750 C in air for LNO electrodes before polarization treatments.

17

Fig. 3 Impedance responses for the oxygen oxidation reaction on LNO oxygen electrodes, measured under open circuit at (a) 600, (b) 650, (c) 700, (d) 750, (e) 800 C, (f) Equivalent circuit.

18

Fig. 4 Activation energy plots of (a) Rp, (b) RH and RL for the oxygen oxidation reaction on LNO oxygen electrodes.

19

Fig. 5 Experimental data and fitting curves of conductivity change of (a) LNO, (b) LSCF as a function of time at 800 C.

20

Fig. 6 (a) Electrochemical impedance spectra, (b) plots of the fitted RP and RΩ of LNO oxygen electrode as a function of time under the anodic current passage of 500 mA cm-2.

21

Fig. 7 XRD patterns of LNO electrodes: (a) as-prepared, after anodic current polarization at 500 mA cm-2 for (b) 8 h, and (c) 12 h.

22

Fig. 8 Core-level XPS spectrum for O 1s peaks of LNO electrodes (a) before and after under anodic current polarization at 500 mA cm-2: (b) 8 h and (c) 12 h.

Table 1 LNO lattice parameters obtained from the refinement by GSAS. Crystal system

Space a（Å）

b（Å）

c（Å）

5.4574

5.4574

12.6672

Rp（%） Rwp（%）

χ2

group

orthorhombic Fmmm

23

2.05

2.65

1.92

Table 2 Binding energies and relative atomic concentrations determined by XPS spectra of O 1s for LNO electrodes before and after testing in air at 750 C under anodic current polarization at 500 mA cm-2. O 1s (Ⅰ)

O 1s (Ⅱ)

O 1s (Ⅲ)

O 1s (Ⅳ)

Peaks Binding energy (eV) 528.85

529.81

531.47

532.96

(20.69%)

(20.48%)

(53.87%)

(4.95%)

528.20

529.33

531.04

532.26

(10.05%)

(43.66%)

(43.19%)

(3.10%)

528.55

529.50

531.39

532.38

(11.33%)

(44.65%)

(36.7%)

(7.32%)

0h

8h

12 h

24