Rate determining steps of fuel oxidation over CeO2 ...

Electrochimica Acta 199 (2016) 108–115

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Rate determining steps of fuel oxidation over CeO2 impregnated Ni-YSZ in H2 + H2O + CO + CO2 ambient D.A. Osinkina,b,* , N.M. Bogdanovicha , A.L. Gavrilyuka,c a b c

Institute of high-temperature electrochemistry, 620137 Yekaterinburg, 20 Academicheskaya st., Russia Ural Federal University, 620002 Yekaterinburg, 19 Mira st., Russia N.N. Krasovskii Institute of Mathematics and Mechanics, 620990 Yekaterinburg, 16 S. Kovalevskaya st., Russia

A R T I C L E I N F O

A B S T R A C T

Article history: Received 9 November 2015 Received in revised form 21 March 2016 Accepted 23 March 2016 Available online 24 March 2016

Polarization resistance of the Ni-Zr0.84Y0.16O1.92 anode impregnated with cerium oxide in H2 + H2O + CO + CO2 + Ar gas mixtures of different composition was studied at temperatures of 700–950 C by means of EIS and subsequent DRT and NLLS analysis of EIS spectra. The electrochemical activity of the anode in the H2 + H2O + CO + CO2 + Ar was found to be defined basically by the hydrogen-containing components (H2 and H2O). The EIS spectra analysis revealed that the anode polarization resistance is determined by three processes, registered at high-, middle- and low-frequencies, respectively. The high-frequency resistance had an activation energy of about 0.93 eV and it did not depend on the gas mixture composition. Both the middle-frequency resistance (activation energy of about 1.04 eV) and the lowfrequency resistance (0.31 eV) depended on the gas atmosphere composition. Suggestions about the nature of the rate-determined steps were proposed. ã 2016 Elsevier Ltd. All rights reserved.

Keywords: SOFC Ni-YSZ anode CeO2 impregnation EIS DRT partial polarization resistance rate determining steps

1. Introduction Solid oxide fuel cells (SOFCs) are a promising technology for energy conversion system [1]. The scientific interest in the SOFC was aroused because of the possibility of using different kinds of fuels, including hydrocarbons and products of their conversion. Utilization of a hydrocarbon fuel is possible due to the high operational temperature of the SOFC (700–1000 C) and the high catalytic activity of the Ni-containing anode. It is known [2] that using hydrocarbons as fuel is less effective than using H2 + H2O gas mixtures because of the higher polarization resistance of the anode. One of the ways to increase the anode activity is to introduce into it fine electrocatalytic powders (metals: Pt, Ru, Ni and/or oxides with mixed conductivity) [3,4]. Introduction of oxides with mixed conductivity, for example CeO2, into the anode by means of wet impregnation (infiltration) can significantly

* Corresponding author at: Institute of high-temperature electrochemistry, 620137 Yekaterinburg, 20 Academicheskaya st., Russia. Tel.:+ 8 343 362 3394. E-mail addresses: [email protected] (D.A. Osinkin), [email protected] (N.M. Bogdanovich), [email protected] (A.L. Gavrilyuk). http://dx.doi.org/10.1016/j.electacta.2016.03.133 0013-4686/ ã 2016 Elsevier Ltd. All rights reserved.

increase the extent of the three-phase boundary electrolyte/ electrode/gas, change the nature of the rate-determining steps [5] and increase the power of the SOFC [6]. For instance, in [7,8] a mechanism of methane electro-oxidation at the anode after ceria was introduced into it was suggested. It was proposed [9,10] that methane oxidation with formation of CO and H2 occurs directly at the ceria surface. The ceria activity for reforming dry methane is determined by its ability to absorb and evolve oxygen in wide ranges of PO2 and temperatures [11]. Utilizing hydrocarbons directly as fuel is not always justified because of carbon deposition at the Ni-containing anode. That is why hydrocarbon conversion or oxidation products are used more often. Gas mixtures obtained by hydrocarbon conversion are of great interest because the ratio of components in the gas mixture can be modified by changing the conversion degree, i.e. the amount ratio of carbon to oxidizer. Work [12] can be mentioned as an example in which it was found that the =2 + E? + E?2 + =2? gas mixture formed by partial electrochemical oxidation of methane results in higher electromotive force [13] in comparison with full oxidation. A similarity in the anode process taking place in cases with either hydrocarbon or hydrogen fuel usage was reported in [14] and the authors conclude that the polarization conductivity of

D.A. Osinkin et al. / Electrochimica Acta 199 (2016) 108–115

the electrode depends only on the partial pressure of oxygen in the gas mixture and not on the fuel type (CO + CO2, H2 + H2O, CH4 + H2O). The electrochemical characteristics of the Ni-cermet electrodes, especially after impregnation, in H2 + H2O + CO + CO2 gas mixtures have still not been studied. It should also be taken into account that the impregnated Ni-cermet is a complex material with a large number of interphase boundaries. A variety of consecutive and parallel heterogeneous reactions can proceed with such materials in H2 + H2O + CO + CO2 gas mixtures [15]. In this paper we investigated the behavior of the polarization resistance of ceria impregnated Ni-Zr0.84Y0.16O1.92 anodes in the H2 + H2O + CO + CO2 + Ar gas mixtures and made qualitative arguments about the nature of the rate determining steps of the electrode process. 2. Experimental 2.1. Samples preparation Commercial NiO and Zr0.84Y0.16O1.92 (YSZ) (production of the Ural Plant of Chemical Reagents, purity not less than 99 wt. %) powders with specific surface areas of 6.9 and 2.3 m2 g1, respectively, were used for the preparation of the anodes. A composite powder of the 56%NiO + 44%Zr0.84Y0.16O1.92 (wt. %) was prepared by a ceramic technique in a planetary ball mill (PM 100, Retsch) and was then sintered at 1200 C for 5 h. A slurry was prepared by mixing the electrode material powder with isopropyl alcohol and polyvinyl butyral. The slurry was painted onto the parallel sides of a dense (about 95% of the crystallographic density) plate of a commercial YSZ electrolyte. After drying, the electrodes were sintered at 1250 C for 2 hours in air atmosphere. The resulting electrode had a thickness of approximately 40 mm and approximate dimensions of 0.5 0.5 cm. The electrode reduction was performed in a humid hydrogen atmosphere for 1 hour at 900 C. The anode composition after reduction was 50%Ni + 50%YSZ (Ni-YSZ). In the last stage of the samples’ preparation the reduced anode was impregnated by a saturated solution of cerium nitrate. Then the samples were annealed at 600 C in a humid hydrogen atmosphere for 1 hour. Weighing the samples before and after impregnation showed that the specific mass of introduced cerium oxide was approximately 1 mg cm2.

109

the current of the electrolyzer so as to get the necessary CO/CO2 ratio. The second sensor was placed into the operating furnace in close proximity to the studied samples. In the H2 + H2O + CO + CO2 + Ar atmosphere the oxygen partial pressure (PO2) is determined by the ratio of reducing components to the oxidizing ones PH2/PH2O and PCO/PCO2. The PO2 will be the same for the same ratios of PH2/PH2O and PCO/PCO2 at equilibrium composition of the H2 + H2O + CO + CO2 gas mixture. It is the PO2 that was measured by the second oxygen sensor. 2.3. Measurements The BET method (in a SORBI N4.1 device) was used for measuring the specific surface area of the powders. Before measuring the specific surface area degasification of powder in a helium atmosphere at 200 C for 1 h was carried out. The powder diffractograms were obtained using an X-ray Rigaku D/MAX2200VIPC diffractometer. In order to obtain SEM micrographs, epoxy was used to fill the pores of the samples in a vacuum with subsequent polishing in a Struers Labopol device. The microphotographs were taken using a scanning electron microscope TESCAN MIRA 3 LMU. The electrochemical characteristics were studied by means of impedance spectroscopy in the frequency range of 105–102 Hz, using a Frequency Response Analyzer FRA-1260 combined with an electrochemical interface EI-1287 (Solartron Instruments Inc.). AC measurements were carried out in potentiostatic mode with a voltage amplitude of 20 mV and 20 points per decade (log scale), in the temperature range 700–950 C. Electrochemical experiments was carried out with five identical samples placed in a measurement rig. A schematic drawing of the measurement rig with the installed samples was given in [16]. Characteristics of three central samples were taken into account. Their electrical data were very close to each other. The measurement rig was connected to the electrochemical interface in a two-electrode four-wire mode which permits the exclusion of the impedance of currentsupplying cables from the overall impedance. Pt mesh and wires were used as current collectors and leads. 3. Results and discussion 3.1. Samples characterization

2.2. Gas mixtures The investigations were performed in H2 + H2O + Ar, CO + CO2 + Ar and H2 + H2O + CO + CO2 + Ar gas mixtures of different composition. For the preparation of the H2 + H2O + Ar high purity gases H2 and Ar (not less than 99.99 vol. %) were used. The amount of vapor in the gas mixture was regulated by controlling the temperature of a saturator through which the H2 + Ar flow was passing. For the formation of the CO + CO2 + Ar mixture, carbon dioxide from the gas bottle was passed through a high-temperature solid oxide electrolyzer operating at 900 C. By changing the value of applied current in the electrolyzer, the CO/CO2 ratio was changed. The obtained gas mixture CO + CO2 was mixed with the correct amount of argon at the output of the electrolyzer. The H2 + H2O + CO + CO2 + Ar gas mixtures, which imitated the products of propane partial oxidation (H/C at. ratio about 8/3), was obtained by mixing CO + CO2 with H2 + Ar and passing this mixture through a saturator. A method to calculate the equilibrium gas composition is given in [16,17]. The flow rate of H2, CO2 and Ar was adjusted and measured by means of Bronkhorst mass flow controllers. Two solid oxide potentiometric oxygen sensors were used in order to control the gas mixture composition. One of the sensors was placed in the electrolyzer to measure the oxygen activity in the CO + CO2 mixture. By knowing the oxygen activity, it is possible to adjust

A diffractogram of the NiO-YSZ powder is shown in Fig. 1. It can be seen from the figure that only diffraction peaks specific to NiO and YSZ are observed. The micrographs of the electrodes before and after impregnation with ceria are shown in Fig. 2. It is easy to distinguish between the phases of nickel and ceramics considering the micropores in the nickel grains which are formed in the course of reduction of nickel oxide. After introducing highly dispersed particles of ceria into the porous electrodes all small voids become indiscernible as they are filled with ceria. However, no separate particle of ceria can be seen, only their agglomerates are

Fig. 1. X-ray diffraction patterns of the NiO-YSZ powder.

110


Fig. 2. Microphotographs of Ni-YSZ electrode before (A) and after (B and B1) impregnation with CeO2 and cross-section of the symmetric Ni-YSZ/YSZ/Ni-YSZ cell (C).

distinguishable. In [18,19] it was shown that, depending on the composition and concentration of the solutions as well as the conditions of their decomposition, the microstructure of the impregnated electrode can be different, as in our case when no individual particles can be seen.

t / °C 10

900

850

800

750

700

before imp. imp. Ni imp. CeO2

R η / Ω cm 2

3.2. Polarization resistance Temperature dependences of the polarization resistance (hereinafter, Rh) of the anode in its initial condition and after its impregnation with ceria in an atmosphere of humid hydrogen are presented in Fig. 3, in the form of Arrhenius-type plots. Obviously, the introduction of ceria results in a significant decrease in Rh (from 0.44 to 0.06 V cm2 at 950 C and from 7.25 to 0.18 V cm2 at 700 C). The attained activity of the Ni-YSZ anode in its initial state is close to the activity of an anode manufactured by similar methods developed by other authors, for example [20,21]. After impregnation of the electrode with CeO2 its activity became higher than that of Ni-YSZ produced by high-technology techniques, e.g. plasma spraying [22]. The temperature dependence of Rh before and after impregnation should be noted. The linear logRh versus inverse temperature dependence is typical for the initial electrode, with an activation energy of approximately 1.16 eV. After introducing ceria, the

950

1

0.1

0.80

0.85

0.90

1000 T

0.95 -1

/K

1.00

1.05

-1

Fig. 3. Temperature dependence of polarization resistance (Rh) of the Ni-YSZ electrode in wet hydrogen, before and after impregnation with Ni and CeO2.


t / °C 950

R η / Ω cm 2

10

900

850

800

750

700

A

1HC 1H 1C calculated for red / ox = 1.5

1

2HC 2H 2C calculated for red / ox = 0.3

0.1 0.80

0.85

0.90

0.95 -1

1000 T / K

1.00

1.05

-1

PCO2 = 0.1 atm (0.39 ± 0.01)

B

PCO = 0.1 atm (0.11 ± 0.01)

R η / Ω cm 2

PCO/PCO2 = 0.63/0.37 (0.59 ± 0.01), PCO plotted on the X-axis

1

PH2O = 0.05 atm (0.39 ± 0.01)

0.1

PH2 = 0.1 atm (0.25 ± 0.02) PH2/PH2O = 0.6/0.4 (0.41 ± 0.02), PH2 plotted on the X-axis

0.1

1

PH2, H2O, CO2, CO / atm Fig. 4. Temperature (A) and concentration (B, 900 C) dependence of polarization resistance (Rh). The reaction orders with respect to the partial pressures (Eq. (1)) are given in parentheses.

aforementioned dependence becomes non-linear. Two straight parts with different slopes can be distinguished in the Arrhenius plot: the first one in the temperature range of 800–950 C and the second one in the 700–800 C, corresponding to activation energy values 0.2 eV and 0.6 eV, respectively (the reasons for the change in the slope of the Arrhenius plot will be discussed in Section 3.6). The authors of [22,23] also pointed out the decrease in activation energy after impregnation of the electrodes. One of the possible reasons for such behavior of the anodes is the high electronic conductivity of ceria in reducing atmosphere. The introduction of ceria leads to extension of the length of the electrode/electrolyte/gas triple phase boundary. However, the broadening of the electrode reaction area in itself cannot cause such excessive changes in the activity of the anodes and in the slopes of the Arrhenius plots for Rh. To prove this assertion, the temperature dependence for Ni-YSZ anode impregnated with a solution of nickel nitrate is shown in Fig. 3 (the impregnation conditions were similar to those described in Section 2.1). It can be seen that the additional introduction of nickel into the electrode does not change the slope of the Arrhenius plot and only slightly decreases Rh, which is caused by an increase in the triple boundary length. Thus, there are serious reasons to suppose that the

111

introduction of ceria not only changes the electrode morphology but completely transforms its electrochemical nature. The temperature dependences of Rh under different gas mixtures are given in Fig. 4A. The gas mixture compositions and their codes are listed in Table 1. It can be seen that the values of Rh in the H2 + H2O + Ar and the five-component mixtures are close to each other, whereas Rh is significantly higher in the CO + CO2 + Ar mixtures. An inverse dependence between Rh and the ratio of the sums of volume percentages for the reducing and oxidizing components, i.e. (%H2 +%CO)/(%H2O +%CO2) (red/ox), is observed for all data obtained. It is worth noting that for the gas mixtures with a red/ox ratio of approximately 1.5 (Table 1) the Arrhenius plots for Rh are nonlinear and their slope decreases in the high temperature region. In the mixtures with red/ox ratio of approximately 0.3 this non-linearity is less notable. Additionally, the calculated dependences of the inverse values of the conductivity sum for the H2 + H2O + Ar and CO + CO2 + Ar gas mixtures are presented in Fig. 4A. Considering that the oxidation reactions of H2 and CO occur independently, the calculated dependences should coincide with the results obtained in the H2 + H2O + CO + CO2 + Ar gas mixtures. However, as can be seen, this is not so. In the case of the gas mixtures with red/ox ratio equal to 1.5 the calculated dependence lies significantly lower than the experimental data. In the mixtures with red/ox ratio of approximately 0.3 at high temperatures it lies also lower whereas at low temperatures it almost coincides with the experimental dependence. One of the possible reasons for such behavior is the partial blocking of the anode reaction centers by tightly bounded adsorbed water in different forms or hydroxyl ions [16]. Detailed study of the influence of the individual components of the gas mixtures on the electrode characteristics is difficult, due to the basic impossibility of effecting a minor change in the percentage of any component without changing the composition of the entire mixture. For this reason, ternary gas systems of H2 + H2O + Ar and CO + CO2 + Ar were tested separately in this work. The concentration dependence of the polarization resistance of the electrodes is presented in Fig. 4B. All obtained dependences are linear in double logarithmic plot. Thus, the dependence of Rh on partial gas pressure can be presented as follows: Rh ¼ Pxred Pyox

ð1Þ

where x and y are the reaction orders. They are determined from the slopes of the straight lines in Fig. 4B. At constant partial pressure, for example, of H2O, Rh is determined by the hydrogen partial pressure Rh P=2x whereas at constant P=2 the polarization resistance is determined by the partial pressure of H2O i.e. Rh P=2?y. At constant P=2/P=2? ratio (PO2 = const) Rh is determined by the contribution of both components. As is shown in Fig. 4B, all obtained dependences of the polarization resistance on partial pressure are different for different gas mixture compositions. In a H2 + H2O + Ar mixture a very weak dependence on the partial pressure of the reducing component, hydrogen (reaction order of 0.1), and a very strong dependence on the H2O partial pressure are revealed. For the Table 1 Equilibrium compositions of gas mixtures, their codes and ratio of sums of percentages for the reduced and oxidized components (rounded to one decimal place). composition of gas mixture/vol. %

code

red/ox

16.5%H2 + 12%H2O + 13.5%CO + 8%CO2 + 50%Ar 16.5%H2 + 12%H2O + 71.5%Ar 13.5%CO + 8%CO2 + 78.5%Ar 5%H2 + 23.5%H2O + 5%CO + 16.5%CO2 + 50%Ar 5%H2 + 23.5%H2O + 71.5%Ar 5%CO + 16.5%CO2 + 78.5%Ar

1HC 1H 1C 2HC 2H 2C

1.5 1.4 1.7 0.3 0.2 0.3

112


example, of three spectra for different gas compositions (Fig. 5). The analysis of spectra was performed using EQUIVCRT [24] software employing the equivalent circuit Rs(RhQh)(RmQm)(RlQl), where Rs denotes the cell series resistance/Ohm; Q denotes a constant phase element/S sn (n is the constant phase exponent/ dimensionless) [25]; subscripts h, m and l indicate the impedance element corresponding to the high-, middle-, and low-frequency process. The impedance spectrum obtained as a result of fitting as well as the partial spectra also are shown in Fig. 5. These discrepancies are related with the spectra fitting error, which reached 10% of magnitude in some cases. The frequencies fmax corresponding to -Z“ maximum, indicated on the partial spectra, were calculated using the equation: pffiffiffiffiffiffiffi n RQ ð2Þ f max ¼ 2p

Fig. 5. Impedance spectra (circles) and fitting and partial spectra (solid lines) for 950 C in different gas mixtures.

CO + CO2 + Ar gas mixtures the inverse dependence is typical, namely a very weak dependence of Rh on the partial pressure of the oxidized form (CO2) and a strong dependence on the partial pressure of CO at constant PCO2. The considered dependences of Rh on temperature and the gas mixture composition characterize the electrode reaction behavior in total. In order to describe the individual steps of the electrode reactions it is necessary to analyze the impedance spectra. 3.3. EIS spectra analysis Let us describe an approach for analyzing the impedance spectra and determining the partial polarization resistance, for the

In recent years DRT (distribution of relaxation times) analysis has been more often employed as an independent or auxiliary analysis for the impedance data treatment [26–28]. In the present work a program code, developed by the authors of [29], based on Tikhonov’s regularization was applied for DRT analysis.The DRT plots for the imaginary parts of the impedance spectra of the anode impregnated with ceria are shown in Fig. 6. Each plot contains three peaks which correspond to the rate-determining steps of the electrode reaction. The high-frequency peaks are registered near 1 kHz, the middle-frequency peaks near 30 Hz (for 1HC and 1H gas mixtures) and 10 Hz (for 1C), the low-frequency peaks near 1 Hz. The DRT dependences for the initial Ni-YSZ anode and anodes after impregnation with nickel in an atmosphere of humid hydrogen are also presented in Fig. 6, as an inset. Clearly, a type of DRT dependence that is different from the DRT dependences of anodes impregnated with ceria can be seen. The number of the peaks, their intensity, width and frequency at which they are registered, change. The highest and most broad peak for the initial Ni-YSZ anode occurs in the range of 20 Hz–20 kHz, i.e. electrochemical processes limiting the electrode reaction rate become apparent just at this frequency domain. Impregnation of the anodes with nickel makes it possible to narrow the frequency range a little due to a decrease in the contribution of the peak registered in the range

Fig. 6. DRT plots for the imaginary part of the impedance of the impregnated with CeO2 electrode, for different gas mixtures at 950 C. Inset Similar DRT plots for the initial Ni-YSZ electrode and for the Ni-YSZ electrode after impregnation with Ni, for wet hydrogen at 950 C.


113

slopes of the straight lines of the Arrhenius-type plots also have close values, approximately equal to 0.93 eV. The concentration dependence of Rh at 900 C for different gas mixtures is presented in Fig. 7B. The Rh values are seen to be close to each other in all gas mixtures being studied. The Rh weak dependence on the gas components concentration (low orders of reactions) is to be noted as well. As a rule, the high-frequency process is ascribed to the charge transfer step [20,21,30–33] which is characterized by a strong temperature dependence of Rh, with activation energy 1.5 to 2 eV, and a value of double layer capacitance of about 20 mF cm2. Our study revealed different high frequency characteristics, namely lower activation energy and much higher capacitance value. The latter was calculated by Eq. (3), being 900 C equal to about 1.5 mF cm2, and did not practically depend on the composition of the gas phase H2 + H2O + CO + CO2 + Ar. Thereby the charge transfer step can definitely be eliminated from consideration of the nature of the high frequency process. pffiffiffiffi npffiffiffi ð3Þ C ¼ 1n R n Q

Fig. 7. Temperature (A) and concentration (B, 900 C) dependence of the highfrequency partial polarization resistance (Rh). The activation energy (in eV) and the reaction orders (Eq. (1)) are given in parentheses on the corresponding graphs.

of 7–15 kHz. After introducing ceria in the anode the electrode process shifts to the low frequency domain, and the electrode process registered at highest frequencies becomes the least pronounced. It is worth mentioning that similar frequency dependences with frequencies corresponding to the maxima of the imaginary part of impedance close to those registered in this work were obtained in [20] for the Ni-YSZ anode (102 –103 Hz). A comparison of the DRT dependences before and after anode impregnation with ceria proves once again the above claim that the introduction of ceria significantly changes the mechanism of the electrode reaction occurring at the Ni-YSZ anodes. 3.4. High-frequency resistance The dependence of the high-frequency partial polarization resistance (Rh) of the anode on temperature in different gas mixtures is presented in Fig. 7A . As is shown in the figure, the behavior of Rh is the same for all the gas mixtures. All temperature dependences (Arrhenius-type plots) are linear and the Rh values are close to each other. The activation energies calculated from the

Fig. 8. Temperature (A) and concentration (B, 900 C) dependence of middlefrequency partial polarization resistance (Rm). The activation energy (in eV) and the reaction orders (Eq. (1)) are given in parentheses on the corresponding graphs.

114


(at constant PCO) and PH2 (at constant partial pressure of H2O). This fact can point to the crucial role of CO and H2O in the current generation process at the anodes. In the case of the H2 + H2O + Ar mixtures equation (1) for the middle frequency resistance is as follows: 0:23 Rm ¼ P0:02 H2 P H2O

ð4Þ

and for the CO + CO2 + Ar mixtures 0:03 Rm ¼ P0:48 CO P CO2

Fig. 9. Temperature dependence of the low-frequency partial polarization resistance (R1) in different gas mixtures. The activation energy (in eV) is given in parentheses.

One of the possible explanations for such behavior of the highfrequency partial polarization dependence may be the finite transport rate of the oxygen ions in the near-surface layer of ceria and/or on its surface, as this process does not depend on the surrounding gas phase composition. Since ceria in reducing atmospheres has both ion and electron conductivity, the electrochemical reaction can occur on its surface at its contact with nickel and YSZ electrolyte. The obtained absolute values of the Rh activation energy are close to the activation energy values of the ion conductivity for the grain boundaries of polycrystalline ceria (0.88–0.93 eV) [34]. Our view regarding the nature of Rh is similar to the opinion of authors [21], who related the relaxation process to the charge transfer (proton) through the Ni/YSZ boundary as well as to the difficulty of oxygen ion transfer through the ceramic constituent of the composite electrode (the investigations were carried out in cells with Ni/YSZ electrodes without impregnation). In addition, the authors stress that the resistance of the high-frequency relaxation process is almost insensible to the gas phase composition (the tests were performed only in H2 + H2O + inert gas atmosphere). In ref. [35] there are highly controversial suggestions related to the nature of the high-frequency process, namely, that it is connected with hydrogen dissociation on the nickel surface and hydrogen transfer from nickel to the ceramic part of the anode near the three-phase boundary (TPB).

ð5Þ

The values of reaction orders for Rm (Eqs. (4) and (5)) indicate that the nature of the corresponding process in the H2 + H2O + Ar and CO + CO2 + Ar gas mixtures is different. Special attention should be paid to the fact that Rm is almost independent of the partial pressure of hydrogen (reducing component) and CO2 (oxidizing component). It is hard to identify clearly the nature of the Rm related process. Considering the frequency for which Rm as well as the reaction orders are registered (Figs. 5 and 6) and value of Cm, charge transfer through the metal/electrolyte boundary and nondissociation diffusion/adsorption should be excluded. In refs. [20,21,30,35–37] a middle-frequency process is connected with reactions on the gas/electrode (adsorption, dissociation, desorption, etc.) boundary and/or with the surface reactions of adsorbed particles. In this regard, it is possible to assume that at the electrodes, after impregnation with ceria, some surface reactions of the adsorbed gas species on the CeO2 particles surface are feasible. This can be proven by the high value of Cm which was approximately 30–70 mF cm2 at 900 C, several orders of magnitude higher than that presented in [20,21,35]. Comparison of the SEM micrographs before and after impregnation (Fig. 2) reveals that the internal electrode surface significantly increases due to the highly-dispersed particles and their agglomerates which results in highly increased values of the Cm capacitance. It is worth noting that the reactions order with respect to hydrogen and carbon dioxide is almost equal to zero. One can assume that these gases either do not participate in the electrode process registered at midfrequencies or do not form active adsorbed species for taking part in the surface reactions. 3.6. Low-frequency resistance The temperature dependence of the low-frequency polarization resistance (Rl) in different gas atmospheres is presented in Fig. 9. The activation energy value for all mixtures is about 0.31 eV. As shown in Fig. 9, Rl is determined in the temperature range 800– 950 C. At temperatures below 800 C Rl has much lower values in comparison with the high- and middle-frequency resistances and it cannot be determined from impedance spectra. At 950 C Rl has its highest value, as Rl decreases with decreasing temperature (high- and middle-frequency polarization resistances exhibit opposite behavior). This temperature dependence of Rl is responsible for the non-linear Arrhenius-type plots for the total

3.5. Middle-frequency resistance The temperature and concentration dependences of the middle-frequency partial polarization resistance (Rm) are shown in Fig. 8. The temperature dependence of Rm, as distinct from that of Rh, can be described as a linear dependence in the Arrheniustype plot with very small error Fig. 8A. The Rm values in the fivecomponent gas mixtures and in the H2 + H2O + Ar mixture at equal red/ox ratio are almost the same; however Rm is significantly higher in the CO + CO2 + Ar mixture. A minor change of activation energy from 0.93 to 1.09 eV is also observed upon changing the hydrogen-containing gas media to CO + CO2 + Ar. Fig. 8B illustrates that Rm is almost insensible to the change of CO2 partial pressure

Table 2 Inter-diffusion coefficient DAB [39] (rounded to one decimal place) for different gas mixtures. gas mixture

H2 + H2O H2 + Ar H2O + Ar CO + CO2 CO + Ar CO2 + Ar

DAB/cm2 s1 700 E

900 E

1100 E

6.5 5.7 1.8 1.2 1.4 1.1

9.0 7.7 2.5 1.6 2.0 1.5

11.7 10.0 3.3 2.1 2.6 2.0


Rh (Fig. 3 and 4A). The Rl value is seen to be significantly higher in the CO + CO2 + Ar gas mixtures than in H2 + H2O + Ar. In addition, all temperature dependences (Arrhenius-plots) correspond to straight lines with positive slopes. Considering this fact as well as the frequencies at which this process is registered (Fig. 5 and 6), one can definitely claim that it has a gas-diffusion nature [21,38,39]. Indeed, in this case Rl is expected to be related with the inter-diffusion coefficient of the gas mixture DAB, according to equation (6) [39]: 2 RT l 1 1 þ ð6Þ Rl ¼ 4F PDAB X A X B where R is the universal gas constant, T is the temperature, F is the Faraday constant, l is the gas-diffusion layer thickness, P is pressure and X is the molar fraction of the component in the gas phase. DAB values for binary gaseous mixtures are given in Table 2 [39]. As is seen, the hydrogen containing mixtures have high DAB. It can be assumed, that the similar behavior is typical for more complex mixtures of gases, i.e. the hydrogen containing gases have the highest DAB, while gases without hydrogen (1C and 2C) have the lowest DAB. Thus, based on eq. (6) at the other equal conditions Rl is the highest in 1C and 2C gas mixtures what is seen from Fig. 9. It also follows from eq. (6) that Rl T2 and DAB1 and taking into account that DAB T1.5 [39], therefore Rl T0.5 which causes the positive temperature dependence of Rl with low activation energy value. 4. Conclusion The behavior of the polarization resistance of a Ni-YSZ electrode impregnated with ceria was studied by means of EIS. The effect of temperature and of the H2 + H2O + CO + CO2 + Ar gas mixture composition on the polarization resistances was investigated. It was revealed that the electrode reaction was limited by three processes registered as: high-, middle- and low frequency. As was expected the high-frequency contribution of polarization resistance was related to the finite transport rate of the oxygen ions in the subsurface layer of ceria and/or on its surface, the middlefrequency one was associated with surface reactions and the lowfrequency one with a process of gas diffusion nature. Acknowledgments This work was partly carried out using facilities of the shared access center ‘Composition of compounds’ IHTE, UB RAS. The research was supported by the Russian Foundation for Basic Research ( 16-03-00434). We are grateful to Boris Kuzin and Dimitry Bronin for his constructive advices on the manuscript.

115

References [1] S.P. Jiang, S.H. Chan, J. Mater. Sci. 39 (2004) 4405–4439. [2] A. Utz, A. Leonide, A. Weber, E. Ivers-Tiffée, J. Power Sources 196 (2011) 7217– 7224. [3] S.P. Jiang, S. Zhang, Y.D. Zhen, W. Wang, J. Am. Ceram. Soc. 88 (2005) 1779– 1785. [4] N. Xu, X. Li, X. Zhao, H. Zhao, K. Huang, Electrochem. and Solid-State Letters 15 (2012) B1–B4. [5] A.P. Khandale, S.S. Bhoga, J. Power Sources 268 (2014) 794–803. [6] D.A. Osinkin, N.M. Bogdanovich, S.M. Beresnev, V.D. Zhuravlev, J. Power Sources 288 (2015) 20–25. [7] B.C.H. Steele, I. Kelly, H. Middleton, R. Rudkin, Solid State Ionics 28–30 (1988) 1547–1552. [8] B.C.H. Steele, P.H. Middleton, R.A. Rudkin, Solid State Ionics 40–41 (1990) 388– 393. [9] K. Foger, K. Ahmed, J. Phys. Chem. B 109 (2005) 2149–2154. [10] N. Laosiripojana, S. Assabumrungrat, Applied Catalysis B: Environmental 60 (2005) 107–116. [11] F. Esch, S. Fabris, L. Zhou, T. Montini, C. Africh, P. Fornasiero, G. Comelli, R. Rosei, Science 29 (2005) 752–755. [12] H. You, A. Abuliti, X. Ding, Y. Zhou, J. Power Sources 165 (2007) 722–727. [13] K. Kendall, C.M. Finnerty, G. Saunders, J.T. Chung, J. Power Sources 106 (2002) 323–327. [14] A. Weber, B. Sauer, A.C. Müller, D. Herbstritt, E. Ivers-Tiffée, Solid State Ionics 152–153 (2002) 543–550. [15] V. Menon, Q. Fu, V.M. Janardhanan, O. Deutschmann, J. Power Sources 274 (2015) 768–781. [16] D.A. Osinkin, B.L. Kuzin, N.M. Bogdanovich, Rus. J. Electrochem. 46 (2010) 41–48. [17] D.A. Osinkin, B.L. Kuzin, N.M. Bogdanovich, Solid State Ionics 251 (2013) 66–69. [18] T.Z. Sholklapper, C.P. Jacobson, S.J. Visco, L.C. De Jonghe, Fuel cells 08 (2008) 303–312. [19] S.P. Jiang, Mat. Science and Engineering A 418 (2006) 199–210. [20] H.P. Dasari, S. Park, J. Kim, J. Lee, B. Kim, H. Je, H. Lee, K.J. Yoon, J. Power Sources 240 (2013) 721–728. [21] S. Primdahl, Nickel/yttria-stabilized zirconia cermet anodes for solid oxide fuel cells, Ph.D. Thesis, University of Twente, Netherlands, 1999. [22] C. Metcalfe, J. Kuhn, O. Kesler, J. Power Sources 243 (2013) 172–180. [23] T. Chen, M. Liu, C. Yuan, Y. Zhou, X. Ye, Z. Zhan, C. Xia, S. Wang, J. Power Sources 276 (2015) 1–6. [24] B.A. Boukamp, Solid State Ionics 20 (1986) 31–44. [25] E. Barsoukov, R.J. Macdonald, Impedance spectroscopy. Theory, Experiment, and application, Wiley, 2005. [26] H. Schichlein, A.C. Müller, M. Voigts, A. Krügel, E. Ivers-Tiffée, J. Appl. Electrochem. 32 (2002) 875–882. [27] B.A. Boukamp, Electrochim. Acta 154 (2015) 35–46. [28] Y. Zhang, Y. Chen, M. Yan, F. Chen, J. Power Sources 283 (2015) 464–477. [29] A.L. Gavrilyuk, D.A., Osinkin, B.L., Kuzin, D.I. Bronin, 19th int. conf. Solis State Ionics (2013) Mon-E-087. [30] S. Primdahl, M. Mogensen, J. Electrochem. Soc. 144 (1997) 3409–3419. [31] V.N. Chebotin, I.D. Remez, L.M. Solovieva, S.V. Karpachev, Electrochim. Acta 29 (1984) 1380–1388. [32] R.D. Armstrong, B.R. Horrocks, Solid State Ionics 94 (1997) 181–187. [33] N.L. Robertson, J.N. Michaels, J. Electrochem. Soc. 138 (1991) 1494–1499. [34] X. Guo, R. Waser, Prog. Mater. Sci. 51 (2006) 151–210. [35] S.P. Jiang, S.P.S. Badwal, Solid State Ionics 123 (1999) 209–224. [36] J. Mizusaki, H. Tagawa, T. Saito, T. Yamamura, K. Kamitani, K. Hirano, S. Ehara, T. Takagi, T. Hikita, M. Ippommatsu, S. Nakagawa, K. Hashimoto, Solid State Ionics 70–71 (1994) 52–58. [37] C. Jin, C. Yang, F. Chen, J. Electrochem. Soc. 158 (2011) B1217–B1223. [38] S. Primdahl, M. Mogensen, J. Electrochem. Soc. 145 (1998) 2431–2438. [39] D.A. Osinkin, B.L. Kuzin, N.M. Bogdanovich, Rus. J. Electrochem. 45 (2009) 483–489.