铬酸钇YCrO3固体氧化物燃料电池阳极材料的合成和性能初研究_图文

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5、rial may be protected by copyright law (Title 17 U.S. CodeECS Transactions, 57 (1 1479-1489 (201310.1149/05701.1479ecst ©The Electrochemical SocietyH 2 Oxidation on Doped Yttrium Chromites Anode of Solid Oxide Fuel CellWenyuan Li, Mingyang Gong, and Xingbo LiuMechanical & Aerospace Engineer

6、ing Department, West Virginia University,Morgantown, WV 26505, USADoped yttrium chromites as potential anodes for SOFC are studiedwith respect to the electrode performance and anode reactionmechanisms. Both electrical conductivity and electrodeperformance of doped yttrium chromites have been enhance

7、d afterNi doping. Electrochemical impedance spectra results indicate thatcharge transfer process at high frequency and surfaceadsorption/diffusion processes at low frequency domain can be thedominant anode reaction steps. Ni doping accelerates the surfaceprocesses and improves the charge transfer pr

8、ocess probably byincreasing the amount of adsorbed H on the electrode surface. Amodel of H2 oxidation reaction is proposed, revealing thisdependence stems from the reaction between adsorbed H and thelattice oxygen.IntroductionIn the state-of-the-art SOFC, Ni/YSZ cermet is the most commonly used anod

9、e. However, it suffers performance degradation when feeding hydrocarbon and impurity containing syngas due to coking and poisoning by contaminants such as S (1. To overcome this drawback, ceramic anodes including oxides with perovskite structure such as doped lanthanum chromites (2-5 and lanthanum d

10、oped strontium titanate have been proposed as potential anode alternatives (6-9. YCrO3 based materials have been evaluated by Yoon et al. as a potential anode after doping with Ca and Co at the A and B sites separately. Compared with other ceramic anodes, YCrO3 shows several advantages: 1 its coeffi

11、cient of thermal expansion (CTE is close to YSZ, which is more compatible with the Ni/YSZ system; 2 the electro-catalytic properties of YCrO3 can be improved by B-site doping; and 3 unlike some anodes such as (La, SrVO3, it can be manufactured in the air. Preliminary results have shown promising per

12、formance and good S-tolerance of YCrO3-type anodes (10, but to the best of our knowledge, there have not been any systematic investigations on anode reaction mechanisms reported so far.Exploration of the electrode reaction mechanism is crucial to guiding the design of materials to improve electrode

13、performance. Although mechanisms and kinetics research based on Pt and Ni anodes have been extensively investigated, the same for perovskite based anodes is scarce (11-19. To this end, Co and Ni doped YCrO3-YSZ composite anodes in this work were developed and characterized in H2-containing atmospher

14、es by electrochemical impedance spectra (EIS. The effect of doping on catalytic activity and anode performance was evaluated. The rate limiting steps and H2 dependence of polarization resistance associated with different dopants were determined. Finally, a model concerning anode reaction mechanism w

15、as proposed based on these results.ExperimentalY 0.8Ca 0.2CrO 3 (YCC, Y0.8Ca 0.2Cr 0.8Co 0.2O 3 (YCCC, and Y0.8Ca 0.2Cr 0.9Ni 0.1O 3(YCCN were synthesized by EDTA-citric sol-gel method detailed elsewhere (20-21. After dissolving standard nitrates (Alfa Aesar in stoichiometric percentage along with c

16、itric acid (Alfa Aesar and dissolving EDTA powders (Fisher Scientific into two beakers of distilled water, they were blended together followed by adjusting the pH to 8 via ammonia water (Alfa Aesar, then held at 80 o C and stirred until gelation. The gel was heated to 400 o C and the resultant powde

17、rs were calcined at 1200 o C for several cycles with intermediate grinding between cycles. X-ray diffraction (XRD, Bruker AXS, Cu K radiation tests were conducted to examine the purity of the calcined powder. Commercial software Jade 5 was used to analyze the XRD spectra. Powders were also pressed i

18、nto pellets for scanning electron microscopy (SEM, JEOL JSM7600F observation.The anode slurry was made by grinding sintered powders with YSZ in ink vehicle. The weight ratio of the anode powders: YSZ: vehicle was 4:6:11. Electrodes were made by screen printing the anode slurry onto both sides of the

19、 YSZ electrolyte symmetrically. YSZ pellets (Nextech Co., 28 mm in diameter and 350 m in thickness, were used as the electrolyte. The active electrode area was 0.7 cm2 at each side. The as-made symmetric cells were sintered at 1000 o C for 2 h in air. Pt mesh was used as current collector.EIS was em

20、ployed to characterize these symmetric cells in various H2-containing atmospheres by Solartron 1287 electrochemical interface and 1260 impedance analyzer at open circuit condition (OCV over the frequency range from 0.1 Hz to 1MHz. The AC signal applied was 20 mV. H2 content in the H2/N2 mixture was

21、controlled by mass flow controllers (Alicat Scientific. A water bubbler at room temperature was used to humidify mixed gases before feeding them to the samples. The resultant spectra were deconvoluted using Z-view software.Results and Discussion It has been reported by K. J. Yoon etc. that a pure ph

22、ase of Ca and Co co-doped YCrO 3 was obtained by glycine-nitrate method at 1200 o C (10. Fig. 1 shows the XRD spectra from the powder samples of pristine and doped YCCs made by sol-gel method in this paper. All peaks correspond to a single phase, showing orthorhombic perovskite structure (PDF#48-047

23、4. The indexes of main peaks have been identified.( 2 11(002(120 (022 (122 (111I n t e n s i t y (a . u . (311 (112 (131 (213 ( 2 (331 (422 YCCN YCCCYCC203040506070802T (degFigure 1. XRD spectra of doped YCCsFigure 2. SEM cross-sectional view of YCCN electrode sintered at 1000 o C (a; morphology of

24、YCCC (b, YCC (c, and YCCN (d sintered at 1200 o C.ECS Transactions, 57 (1 1479-1489 (2013The microstructure and morphology of these powders and the YCCC electrode were examined by SEM. It is indicated in Fig. 2a that the microstructure is uniform and there is enough porosity for gas transportation i

25、n the YCCC electrode sintered at 1000 o C. The adhesion between electrode and YSZ electrolyte is good and no delamination at the interface was observed. Shown in Figs. 2b to 2d, the grain size sintered at 1200 o C is 0.20.5 m for YCC, 11.5 m for YCCC and 0.20.5 m for YCCN respectively. (a-Z ' &#

26、39; (: c m 2 2Z' (: cm Figure 3. Impedance spectra of YCCs at OCV tested in wet 5% H2N2 at 850 o C (a and the equivalent circuit used to fit EIS (b.Fig. 3a shows the EIS for YCCs tested at 850 o C in wet 5% H2-N 2 (5% H2 and 95% N 2. The scattered symbols are measured data and the solid lines ar

27、e the corresponding fitting arcs from Z-view program. Fig. 3b is the equivalent circuit used to fit the spectra, where L represents the overall inductance from the lead wires and the Solartron system, R 0 the total ohmic resistance composed of contributions from electrolyte, electrodes and lead wire

28、s. A constant phase element (CPE is adopted due to the frequency dispersion phenomenon in the electrode process (22. As can be seen in Fig. 3a, the chosen equivalent circuit demonstrated a good representation of the observed results. Two main arcs are distinguishable in these spectra, which are deno

29、ted as high frequency (HF and low frequency (LF arcs separately in the frequency domain. All of these spectra possess roughly similar shapes, indicating that the main electrode processes remain the same after doping even if the performance of YCC has been enhanced significantly. (al n (T /R 1000/T (

30、1/K (bl n (T /R 1000/T (1/KFigure 4. Arrhenius curves for HF arc (a and LF arc (b derived from EIS of YCCs in wet 5% H2N2 from 650850 o CFig. 4 demonstrates the Arrhenius curves for HF and LF arcs derived from EIS of YCCs in 5% wet H2. It is obvious that resistances from different samples decreased

31、in the order R YCC > R YCCC > R YCCN for both HF and LF arcs. The apparent activation energy (E a derived from Fig. 4a for the HF arc is almost the same, with values of 1.2, 1.3 and 1.2 eV for YCC, YCCC and YCCN, respectively. In contrast, for LF arcs, E a varies significantly for different ma

32、terials, with 1.2, 1.0 and 0.5 eV for YCC, YCCC and YCCN, respectively. Table I summarizes the resistance (R , capacitance (C , activation energy (E a and characteristic frequency (f 0 from EIS collected in wet 5% H2 at 850 o C. After comparing the f 0 and C identified in this study to those reporte

33、d in the literature for SOFC electrodes (23, 24, it is safe to assign the HF arc and LF arc for YCCs in wet 5% H2 to charge transfer and hydrogen adsorption/diffusion processes, respectively.Table I. Summary of resistance (R , capacitance (C , activation energy (E a and characteristic frequency (f 0

34、 for YCCs tested in wet 5% H2-N 2 at 850 o C.In wet 5% H2-N 2at 850 o CR (cm 2C (F Ea (eV f o (HzHF arcLF arcYCC 157 2.0h 10-7YCCC 44 1.4h 10-7YCCN 24 2.3h 10-7YCC 43 0.7h 10-3YCCC 26 1.6h 10-3YCCN 18 3.8h 10-31.2 48001.2 25000 1.3 27000 1.2 1 1.0 4 0.5 2The performance of YCC was significantly enha

35、nced upon Ni and Co doping as shown in Fig. 3a. For this charge transfer process at HF arc region, the reaction rate is controlled by E a and the amount of available reactants. Given that all three materials show similar E a at HF arc, the amount of available reactants on the YCCN and YCCC electrode

36、 can be expected to be higher for the improved charge transfer reaction. Although the ionic conductivity of YCCs is not available so far, it must possess oxygen ion conductivity in some extent especially in a reducing atmosphere (25. The oxygen non-stoichiometry of YCC was reported to be 0.06 under

37、10-18 atm P O2 at 1000 o C (26. The doping of Ni and Co should increase this value due to the fact that perovskite oxides with Ni or Co as B site anion are much easier to reduce than those with Cr at B site (27. Therefore, YCCs can be viewed as mixed ionic and electronic conductor (MIEC in the anode

38、 atmosphere. In this case, the electrode reaction can take place not only at three phase boundaries (3PBs but also at the surface of YCCs grains. The reducibility introduced by Ni and Co doping make it easier to activate lattice O in YCCs to react with H adsorbed on the surface. The higher ionic con

39、ductivity of YCCC and YCCN would also make the supply of O from YSZ faster. Both factors above would accelerate the charge transfer reaction in 2 phase boundaries (2PBs where the gas channel and MIEC meet. From the viewpoint of reactant H, the coverage of adsorbed H may be increased by doping. In th

40、e LF arc, YCCN displayed the best performance. The lowest E a of LF arc implies the adsorption/diffusion process is thermodynamically more favorable on the YCCN sample. High stick coefficient of metal Ni for H adsorption has been reported by Morgensen et al. (12. Good H adsorption/diffusion capabili

41、ty of the Ni-based anode in SOFC is also well-known. Based on this similarity, there might be some correlation between YCCN and metal Ni at this point.EIS testing was carried out in a H2 partial pressure range from 10-1.5 to 1 atm on YCCN and YCCC electrodes (not the same samples as used in Fig. 3.

42、The double logarithm plots in Fig. 5 show the P H2 dependence of HF arc in different temperatures.ECS Transactions, 57 (1 1479-1489 (2013The reaction order n in the relationship of 1/R P n H2 is the slope of each curve, ranging from 1/3 to 1/2 as marked in Fig. 5a for YCCN and around 1/4 for YCCC. (

43、a l o g (1/R log (PH2/atm(bl o g (1/R log(P H2/atmFigure 5. H2 dependence of HF arc for YCCN (a and YCCC (b in different temperaturesECS Transactions, 57 (1 1479-1489 (2013Partial pressure dependence of anode reaction has been extensively researched in the Ni-YSZ system (23-24, 28-31. Most of the re

44、sults in these references show that the polarization resistance of Ni-YSZ electrode is insensitive to P H2, but strongly dependent on P H2O . According to observations by Mogensen et al. (12, the sticking coefficient of H on Ni increases with temperature, and was about 0.3 at SOFC operating temperat

45、ure even for the most unfavorable crystal face, rendering a high H coverage on Ni surface. If the coverage of adsorbed H on the electrode surface is high enough, such a process would not be affected by H2 molecule content, especially when running EIS at OCV condition. Nonetheless, with lower H cover

46、age, the performance of the YCCs electrode would be harmed by limitation of the H supply when decreasing P H2. In addition, both the ionic conductivity and activation of lattice oxygen are closely related to the content of H2, which play important roles in the charge transfer step as discussed above

47、. Hence, it is not surprising that such electrodes showed strong H2 dependence.The overall H2 oxidation reaction at the anode can be represented in Eq. 1,&H 2 O o lH 2O+VO 2e 1Base on the observed results, one possible model for the reaction path on YCCsanodes in this study can be presented as f

48、ollowing:Step 1 H 2l2H ads, YCCN 2 Step 2 H ads, YCCN lH ads, 3PB 3 -Step 3 H ads, 3PB O O lV O OH e 4Step 4 H ads, 3PB OH lH 2O e 5After taking the following assumptions: 1 these reactions are microscopicallyreversible; 2 adsorption of any species is governed by Langmuir type isotherm; 3 the surfac

49、e coverage for all adsorbed species is low, the corresponding rate equations can be written as:2r 1 k 1P H2 k 1 a Hads 6-r 2 k 2a Hads k 2a H3PB 7r 3 k 3a H3PB exp(EfE k 3 a OH3PB exp( DfE 8r 4 k 4a H3PB a OH3PB exp(EfE k 4P H2O exp( DfE 9where k i and k i - are the rate constants for the correspond

50、ing anodic and cathodic reaction;f =F /RT (F , R and T have their usual physical meaning and E the electrode potential. and are symmetric coefficients. The activity of lattice oxygen and oxygen vacancy is taken as constant regardless of the change of P H2.ECS Transactions, 57 (1 1479-1489 (2013 We f

51、irst assume that the charge transfer step (step 3 is the rate-determination-step (RDS and the other steps are in near equilibrium. It follows: a Hads k1 PH2 k1 10 a H3PB k 2 a Hads k 2 k4 PH2O k 4 a H3PB exp( fE 11 a OH3PB 12 Substituting of Eq. 10 - 12 to Eq. 8, it follows: r3 k 4 k 2 k3 k 4 k 2 k1

52、 PH2 k 4 k2 P H2O exp( E fE k3 k4 k2 k1 k1 exp( (1 D fE k1 PH2 13 Eq. 13 indicates the steady state reaction rate of step 3, which can be converted to current density using the relationship of I = nFr. In equilibrium condition there is no net current, i.e. r3=0, it reveals the Nernst potential: E co

53、nst RT pH2O ln 2F pH2 14 Inserting Eq. 14 back into Eq. 13 yields the relationship of io,3 PH21/4 take =0.5. PH2O at OCV is almost fixed, 0.03 atm, in the present experiment. Similarly, if assuming the second charge transfer step (step 4 the rds, io,4 PH21/4 can be drawn also. Thus, a 1/4 reaction o

54、rder is predicted at high frequency domain. As discussed above, doped YCCs possess some level of ionic conductivity at very low PH2 condition. They are MIEC when working as anodes, and the charge transfer process (step 3 and 4 not only take place at 3PB area but also occur at 2PB area as O ions can

55、be transported through the bulk of YCCs. The activity of oxygen vacancy in step 3 for 2PB reaction is no longer constant, but dependent to PH2. Furthermore, besides H adsorption, O and H2O can be adsorbed on electrode surface as well, leading to a complex competitive adsorption model as studies by I

56、hara and Mizusaki (18, 32. Given all these issues above, the reaction path in the anode may be more complicated than that presented in this model, and the H2 dependence would be more intricate than derived in this study as well. However, to further justify this model and verify the dominant reaction

57、 mechanism, more experiments are needed, such as EIS study under polarization conditions, the effect of H2O on polarization resistance and etc., which are the subjects of upcoming investigations. Downloaded on 2015-04-08 to IP 24 address. Redistribution subject to ECS terms of use (see ec

58、/site/terms_use unless CC License in place (see abstract. 1487 ECS Transactions, 57 (1 1479-1489 (2013 Conclusions In this study, doped YCCs were made and investigated by EIS. The effect of Ni and Co doping on electrode performance was discussed. A possible anode reaction model was proposed b

59、ased on H2 dependence testing. It was found that charge transfer and surface adsorption/diffusion processes could dominate the spectra at high and low frequency domain, respectively. The electrode resistance of YCC was significantly decreased with Ni and Co doping. Ni doping facilitated the surface process in the LF domain by decreasing the