Influence of modifying additives on electrochemical performance of La2NiO4+δ - based oxygen electrodes

Solid State Ionics 346 (2020) 115215

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Influence of modifying additives on electrochemical performance of La2NiO4+δ - based oxygen electrodes

T

E.P. Antonovaa,b, , A.V. Khodimchuka,b, E.S. Tropina,b, N.M. Porotnikovaa,b, A.S. Farlenkova,b, M.I. Vlasova,b, M.V. Ananyeva,b ⁎

a b

Institute of High Temperature Electrochemistry UB RAS, 620137 Yekaterinburg, Russia Ural Federal University, 620002 Yekaterinburg, Russia

ARTICLE INFO

ABSTRACT

Keywords: Modifying additive Lanthanum nickelate Polarization resistance Rate-determining stage

The effect of the La2NiO4+δ and Pr2NiO4+δ modifying additives on the electrochemical performance of the La2NiO4+δ oxygen electrodes has been investigated. It has been shown that the introduction of the additives reduces the polarization resistance of the electrodes. The lowest value of the polarization resistance has been obtained for the La2NiO4+δ electrodes modified with Pr2NiO4+δ, which was 0.44 Ω cm2 at 650 °C in an air atmosphere. The introduction of modifying additives is shown to have a significant impact on the electrode reaction stage associated with the oxygen exchange. Probable reasons for the phenomena are discussed.

1. Introduction One of the research trends in the field of alternative energy is the development of solid oxide fuel cells (SOFCs). Current requirements for SOFCs imply a decrease in the device operation temperature from 900–1000 °C to moderately high temperatures of 500–800 °C. One of the ways to solve this problem is the application of the active electrodes with improved performance in a given temperature range. Materials with a K2NiF4-type structure (Ruddlesden – Popper structure), in particular, lanthanide nickelates Ln2NiO4+δ (Ln = La, Pr) are among the promising materials for creating oxygen electrodes for solid oxide electrochemical devices operating in the medium temperature range. Physicochemical properties of Ln2NiO4+δ oxides, as well as their electrochemical activity, are extensively studied in recent years [1–6]. In particular, it was found that in the row Ln2NiO4+δ (Ln = La, Pr), praseodymium-containing oxide exhibits higher electrochemical activity [5]. However, Pr2NiO4+δ can decompose in oxidizing atmospheres at temperatures below 900 °C, while La2NiO4+δ is stable under these conditions [6]. Therefore, it is of great interest to develop methods for increasing the electrochemical activity of La2NiO4+δ electrodes. Much attention is devoted to the development of electrode manufacturing (varying the deposition techniques and the sintering temperature) in order to increase their efficiency [7,8]. One of the ways to solve this problem is the introduction of modifying additives into the electrode structure [9–11]. The choice of additives in the known methods of electrode modification is based on the selection of additives

⁎

with different chemical compositions, which introduction increases the electrode surface and thereby improves the electrode processes. For example, introducing the praseodymium oxide into the electrode matrix is widely used [11,12]. Due to the formation of nanoscale particles, the specific surface area of the electrode significantly increases. As a result, the rate of the stage of the electrode process associated with the oxygen exchange is accelerated. In this case, the electrochemical activity of the electrodes increases, however, the degradation of the electrodes in time due to coarsening of the electrocatalyst particles is observed [12]. A new approach in this direction can be the use of modifying additives that are isostructural to the electrode matrix, which, therefore, should prevent material degradation over time. In this work, we made an attempt to introduce modifying additives La2NiO4+δ and Pr2NiO4+δ to the electrode material La2NiO4+δ and to study their influence on electrode performance. 2. Experimental The Ce0.8Sm0.2O1.9 electrolyte and the La2NiO4+δ electrode material were prepared according to the citrate-nitrate technology. The final synthesis of the electrolyte powder was performed at 1300 °C for 4 h. Dense electrolyte tablets were sintered at 1650 °C for 5 h. The obtained ceramics had a relative density of 96% of theoretical. For the La2NiO4+δ powder, preliminary synthesis was carried out at 950 °C for 2 h, and the final annealing at 1250 °C for 3 h was performed. According to XRD analysis (D/MAX-2200 RIGAKU conventional

Corresponding author at: Institute of High Temperature Electrochemistry UB RAS, 620137 Yekaterinburg, Russia. E-mail address: [email protected] (E.P. Antonova).

https://doi.org/10.1016/j.ssi.2019.115215 Received 16 July 2019; Received in revised form 23 December 2019; Accepted 25 December 2019 0167-2738/ © 2019 Elsevier B.V. All rights reserved.

Solid State Ionics 346 (2020) 115215

E.P. Antonova, et al.

nickel ions, the oxide phases of La2NiO4+δ (or Pr2NiO4+δ) had to be formed. The content of the additive in the electrode layer after the impregnation procedure was 0.7–0.8 mg/cm2 in both cases. The electrochemical activity of the electrodes was studied by means of impedance spectroscopy (impedancemeter Z-500PRO, Elins) in the temperature range of 600–700 °C in air. The impedance spectra were recorded in the frequency range 0.1–105 Hz with an AC voltage amplitude of 30 mV. Electrode polarization resistance (Rη) was calculated according to:

La2NiO4

Intensity, a.u.

Ce0.8Sm0.2O1.91

R =

20

30

40

50

60

70

Rs ) 2

S

(1)

where Rdc is the low-frequency intercept of the spectrum with the real axis, which corresponds to the cell resistance to direct current, S is an electrode area, and Rs is the high-frequency resistance, which corresponds to the ohmic resistance of the electrolyte. Impedance spectra analysis was performed by the distribution of relaxation times (DRT) method using a software based on the Tikhonov regularization method and developed by the authors [13]. The result of the analysis of the impedance spectrum by the DRT method is the relaxation time distribution function, which gives information about the number of relaxation processes and their relaxation times (frequencies). After electrochemical measurements, Raman spectra from the electrode surface of investigated electrochemical cells were recorded at room temperature using Renishaw Ramascope U1000 equipped with a confocal Leica DM LM microscope, a notch filter, a solid-state laser with λ = 532 nm and a cooled charge-coupled device detector. The laser light of output power 0.5 mW was focused on the samples using a 50× objective. Spectra acquisition time was 150 s.

80

2 , (a) Pr2NiO4 La2NiO4

Intensity, a.u.

(Rdc

Ce0.8Sm0.2O1.91

3. Results and discussion

20

30

40

50

60

70

Fig. 1 presents the XRD patterns for impregnated electrodes obtained in the mode of grazing incidence diffraction with an angle of incidence of 1.5°. It is clearly seen that all the peaks can be assigned to electrolyte and electrode phases. Fig. 2 presents microphotographs of the surface of the initial La2NiO4+δ electrodes, as well as electrodes modified with the La2NiO4+δ and Pr2NiO4+δ additivities. As can be seen, the additives are distributed over the electrode surface as the particles with a significantly smaller size compared with those for the electrode matrix and thus increase the electrode specific surface area. No phase contrast in the backscattered electron mode can be seen, as well as all the elements are equally distributed according to X-ray energy-dispersive microanalysis. Fig. 3 shows a comparison of the impedance spectra for the symmetric electrochemical cells with unmodified and modified electrodes at 650 °C in air. The apparent electrolyte resistance was subtracted from the data set for clarity. Fig. 4 presents the temperature dependences of the polarization resistance of the investigated electrodes. It is obvious, that the modification of the electrodes in both cases leads to an increase in the electrochemical activity of the electrodes, and the effect is more pronounced in the case of the use of the modifying additive Pr2NiO4+δ. Table 1 presents the polarization resistance values for all cases at 650 °C. The introduction of a modifying additive reduces the polarization resistance by 2 and 3 times in the case of La2NiO4+δ and Pr2NiO4+δ, respectively. In order to obtain detailed information about the electrode process and the effect of electrode modification on it, the impedance spectra were analyzed in terms of the DRT method. Fig. 5 depicts the calculated frequency dependences of the distribution of the relaxation times function. It can be seen that the electrode process consists of several stages. The number of peaks on the curve, as well as the corresponding relaxation times, does not change their position regardless of whether a modifying additive is used, which suggests that modification of the electrodes does not change the mechanism of the electrode process. At the same time, the introduction of a modifying additive significantly reduces the contribution of the low-frequency stage, which is associated

80

2 , (b) Fig. 1. XRD patterns obtained in the in the mode of grazing incidence diffraction for electrochemical cells with: (a) - La2NiO4+δ - modified electrode, (b) - Pr2NiO4+δ - modified electrode.

diffractometer in CuKα-radiation (λ(Kα1) = 1.54 Å)), all obtained materials were found to be single phase. Symmetric electrochemical cells La2NiO4+δ|Ce0.8Sm0.2O1.9|La2NiO4+δ were fabricated by a screen printing method. The preparation technique consisted of two steps: deposition of the electrode slurry with organic binding, which prevents the trickling of the slurry, onto the electrolyte, and subsequent sintering at a temperature of 1000 °C for 3 h. The electrode thickness, estimated from electron microscopy data (MIRA 3 LMU, TESCAN), was about 45 μm. Two identical electrochemical cells were prepared and investigated by means of impedance spectroscopy. Then, the procedure of electrode modification was performed by impregnating the porous matrix of the electrode with appropriate solutions. For the preparation of the solutions for impregnation, lanthanum, praseodymium and nickel nitrates were dissolved in isopropyl alcohol. The solutions were mixed in a stoichiometric ratio of La (Pr): Ni as 2: 1. After mixing, the concentration of lanthanum or praseodymium in solutions was 50.0 g/l. The impregnation of the electrode layer was followed by drying in air and annealing at a temperature of 950 °C in air for 30 min. After heat treatment of the solution containing lanthanum (or praseodymium) and 2

Solid State Ionics 346 (2020) 115215

E.P. Antonova, et al.

a

b

c

d

e

f

Fig. 2. Microphotographs of the La2NiO4+δ electrode surface in second electron (a, c, e) and backscattered electron (b, d, f) modes: (a, b) – without modification, (c, d) – with La2NiO4+δ additive, (e, f) – with Pr2NiO4+δ additive.

3

Solid State Ionics 346 (2020) 115215

E.P. Antonova, et al.

1.8

cm2]

1.4

-Z", [

0.8

with the exchange process [14]. In order to get a deeper insight into the reasons for the enhanced electrochemical activity of modified electrodes, we performed Raman spectroscopy analysis of the electrochemical cells after experiments. Fig. 6 shows Raman spectra for the initial electrode (without impregnation) and for electrodes after modifications with La2NiO4+δ and Pr2NiO4+δ. On the spectrum for the initial electrode three peaks at 434, 217 and 87 cm−1 can be well-identified. This is in good agreement with literature data typical for La2NiO4+δ tetragonal structure [15–17]. Slight differences in positions and relative intensities of the peaks could be due to differences in oxygen stoichiometry. Spectrum for electrode after modification with La2NiO4+δ, in general, is of the same shape as for the initial electrode, however additional “wings” appear at 400 and 580 cm−1, which also were previously registered for La2NiO4+δ [17]. An important thing that there is no peak at about 1100 cm−1 characteristic for nickel oxide [18,19] or peaks characteristic for lanthanum oxide [20]. These indicate that lanthanum and nickel nitrates react with La2NiO4+δ electrode during the impregnation procedure forming La2NiO4+δ structure, but not La2O3 and NiO oxides. Thus, we conclude that appearance of the additional “wings” near the peak at 434 cm−1 is a result of the increase of local defectiveness of the La2NiO4+δ electrode and possible changes in oxygen stoichiometry due to impregnation. Completely different shape of the spectrum is observed for Pr2NiO4+δ - modified electrode. It is well-known that Pr2NiO4+δ and La2NiO4+δ have similar structures, and their Raman spectra have to be similar [17], which is not the case for the obtained data. On the spectrum, one can clearly reveal a peak at 1170 cm−1, which is characteristic for NiO [18,19], and a number of broad complex peaks at 564, 419, 268 and 140 cm−1. Taking into account the fact that NiO was formed, one can expect the formation of various forms of praseodymium oxides, like PrO2, Pr2O3, Pr6O11. Then, we assume that these complex peaks are resulted from overlapping of the spectra of NiO, various praseodymium oxides, La2NiO4+δ electrode, and Pr2NiO4+δ phase. Based on the obtained results, it can be concluded that in the case of electrode modification by introducing La2NiO4+δ, the improvement in electrochemical performance may be associated with an increase in the electrode specific surface area. The case of the Pr2NiO4+δ introduction is more complicated. Several possible reasons of the enhanced electrochemical performance can be suggested. For electrodes with mixed ionic-electronic conductivity, including the La2NiO4+δ studied in this work, it is known that the processes associated with the oxygen exchange between the oxide and the gas phase make a significant contribution to the total polarization resistance. Previously it was found that for La2NiO4+δ the exchange process is limited by the dissociative adsorption stage, while for Pr2NiO4+δ at a temperature below 700 °C, the rate-determining stage is the incorporation (see Fig. 7) [21,22]. The dissociative adsorption rate for Pr2NiO4+δ is higher than that for La2NiO4+δ. Therefore, the presence of Pr2NiO4+δ oxide can accelerate the dissociative adsorption process and improve electrode performance. Taking into account the Raman data, the presence of nickel and praseodymium oxides also should be considered. The introduction of these compounds is known to have an impact on electrochemical performance [11,23].

without modification with La2NiO4+ additive

1.6 1.2 1.0 0.6 0.4 0.2

0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

cm2] (a)

Z', [ 1.4

without modification with Pr2NiO4+ additive

cm2]

1.0

-Z", [

1.2

0.6

0.8

0.4 0.2 0.0 0.0

0.2

0.4

0.6

Z', [

0.8

1.0

1.2

1.4

2

cm ] (b)

Fig. 3. Area specific electrode impedance spectra for investigated symmetric electrochemical cells taken at T = 650°С in air: a) cell № 1, b) cell № 2.

cm2])

1.6 1.2

cell cell cell

1 without modification 2 without modification 1 with La2NiO4+ additive

0.8

cell

2 with Pr2NiO4+ additive

log(R , [

0.4 0.0 -0.4 -0.8 -1.2 1.00

1.04

1.08

1.12

1.16

-1

1000/T, [K ]

4. Conclusions

Fig. 4. Temperature dependences of polarization resistance for investigated electrodes in air.

The effect of the modifying additives La2NiO4+δ and Pr2NiO4+δ on the electrochemical activity of the La2NiO4+δ electrodes in contact with

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Solid State Ionics 346 (2020) 115215

E.P. Antonova, et al.

Table 1 Polarization resistance values for investigated electrodes in air at the temperature of 650 °С. Modifying additive

Polarization resistance (Rη) without modifying additive, Ohm∙cm2

Polarization resistance (Rη) with modifying additive, Ohm∙cm2

La2NiO4+δ Pr2NiO4+δ

1.61 1.33

0.81 0.44

7 without modification with La2NiO4+ additive

6

without modification

4000

with La2NiO4+ additive with Pr2NiO4+ additive

I, [a.u.]

DRT g f

5 4 3 2

1

10

100

1000

0

10000

Frequency, [Hz] (a)

500

1000

1500

Raman shift, [cm ] Fig. 6. Raman spectra recorded from the electrode surface of the investigated electrochemical cells.

without modification with Pr2NiO4+ additive

-2 -1 s ])

5 4

log(ra, ri, [atom cm

3 2 1 0

0

-1

6

DRT g f

2000 1000

1 0

3000

1

10

100

1000

10000

Frequency, [Hz] (b)

18 17 16 15 La2NiO4+

14 0.90

Pr2NiO4+

ra

ra

ri

ri

0.95

1.00

1.05

1.10

1.15

-1

Fig. 5. Distribution of relaxation times functions (DRT), calculated from impedance data taken at T = 650 °С in air: a) cell №1, b) cell № 2.

1000/T, [K ] Fig. 7. Temperature dependences of oxygen dissociative adsorption rate (ra), and oxygen incorporation rate (ri) for La2NiO4+δ and Pr2NiO4+δ, рО2 = 0.71 kPa [21,22].

Ce0.8Sm0.2O1.9 electrolyte in the temperature range of 600–700 °C was studied by means of impedance spectroscopy. The modification of the electrodes is shown to have a positive effect on the electrochemical performance of the electrodes studied, reducing the electrode polarization resistance. By DRT analysis it is demonstrated that the modification mostly influences the low-frequency stage of the electrode process. In the case of the Pr2NiO4+δ additive, the effect is more pronounced. For La2NiO4+δ modifying additive the improvement in electrochemical performance is associated with an increase in the electrode specific surface area, while for Pr2NiO4+δ additive the acceleration of the dissociative adsorption process as well as the presence of nickel and praseodymium oxides are suggested as possible reasons for the enhanced electrode performance.

CRediT authorship contribution statement E.P. Antonova: Writing - review and editing. A.V. Khodimchuk: Investigation, Visualization. E.S. Tropin: Methodology. N.M. Porotnikova: Writing - original draft.A.S. Farlenkov: Investigation. M.I. Vlasov: Investigation.M.V. Ananyev: Conceptualization, Funding acquisition.

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Solid State Ionics 346 (2020) 115215

E.P. Antonova, et al.

Declaration of competing interest

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