鉻酸釔YCrO3基固體氧化物燃料電池陽極材料的制備和在H2燃料上的性能研究_圖文

1、Journal of Power Sources 241(2013494e 501Contents lists available at SciVerse ScienceDirectJournal of Power Sources wsourH 2oxidation on doped yttrium chromites/yttriumstabilized zirconia anode of solid oxide fuel cellWenyuan Li, Mingyang Gong, Xingbo Liu *Mechanical and Aerospace Engineering Depart

2、ment, West Virginia University, Morgantown, WV 26505, USAh i g h l i g h t sH 2oxidation mechanisms on Co and Ni doped yttrium chromites were investigated.Charge transfer process at HF and surface adsorption/diffusionprocesses at LF can be the dominant anode reaction steps. Reaction order is 1/4for

3、Co doped and 1/3e 1/2for Ni doped yttrium chromites.a r t i c l e i n f oArticle history:Received 15March 2013Received in revised form 18April 2013Accepted 21April 2013Available online 30April 2013Keywords:SOFCCeramic anodeH 2oxidation mechanisms impedancea b s t r a c tCo and Ni doped yttrium chrom

4、ites as potential anodes for solid oxide fuel cell (SOFCare studied with respect to the electrode performance and anode reaction mechanisms. Both electrical conductivity and electrode performance of yttrium chromites have been enhanced after Co and Ni doping. Electrochemical impedance spectroscopy (

5、EISresults indicate that charge transfer process at high frequency and surface adsorption/diffusionprocesses at low frequency domain can be the dominant anode reaction steps. Ni doping accelerates the surface processes by reducing the related activation energy from 1.2to 0.5eV. It also substantially

6、 improves the charge transfer process probably by increasing the amount of adsorbed H on electrode surface. The resistance of high frequency is found to be dependent on H 2content. The observed reaction order is 1/4for Co doped and 1/3e 1/2for Ni doped yttrium chromites. A model of H 2oxidation reac

7、tion is proposed, revealing this dependence stems from the reaction between adsorbed H and the lattice oxygen.Ó2013Elsevier B.V. All rights reserved.1. IntroductionNi/YSZcermet is the most commonly used anode in SOFC with H 2fed as fuel, but it would degrade in hydrocarbon and impurity con-tain

8、ing syngas due to coking and poisoning by contaminants such as S 1. Therefore, alternative materials are long desired to overcome these drawbacks of Ni/YSZanode. Perovskite oxides such as doped lanthanum chromites 2e 5and lanthanum doped strontium tita-nate, etc. have been investigated extensively a

9、s potential anode ma-terials 6e 9. The ABO 3formula allows not only wide size variety at A and B sites but also different valence combination, making the elec-trical and catalytic properties adjustable 10. YCrO 3perovskite has been examined as possible interconnect material as they display advantage

10、s over the traditional LaCrO 3based materials in terms of the*Corresponding author.E-mail address:(X. Liu.chemical expansion and compatibility with yttrium stabled zirconia (YSZelectrolyte 11e 17. In addition of working as interconnect, YCrO 3based materials have been evaluated

11、 as potential anode after multiple doping at A and B sites:an anode made from Ca and Co doped YCrO 3lately has demonstrated an encouraging performance in H 2, and more importantly, displayed good tolerance towards 20ppm H 2S 18. However to our best knowledge, besides the electrode performance resear

12、ch, no investigation regarding the anode reaction mechanism on YCrO 3based materials has been reported. Under-standing on electrode reaction mechanism is important for the tailoring of material to further optimize electrode performance. In fact, on Pt and Ni-based anodes, mechanism and kinetics stud

13、ies have been extensively investigated 19e 27. In these publications, it is generally recognized that most electrochemical reaction takes place at three-phase boundary (3PBarea and that the electrode perfor-mance is heavily dependent on the microstructure and electrode composition 25,28. It is also

14、known from the EIS studies that theW. Li et al. /Journal of Power Sources 241(2013494e 501495charge transfer process is relatively fast as compared to the adsorp-tion and diffusion processes 29e 31. On the other hand, due to the variation on raw materials, electrode composition, manufacturing condit

15、ions and etc., different conclusions concerning reaction path and rate-limiting steps are also drawn out by different groups. These existent observations from Ni-based anodes could be relevant to the study of YCrO 3based anodes as well.In this work, in order to gain knowledge of anode reaction on YC

16、rO 3materials and evaluate electrode performance, Co and Ni doped YCrO 3e YSZ composite anodes were developed and tested in H 2-containing atmospheres by EIS. The effect of doping on catalytic activity and anode performance was evaluated. The rate-limiting steps and H 2dependence of polarization res

17、istance associated with different dopants were determined. At last a model concerning anode reaction mechanism was proposed based on these results.2. ExperimentY 0.8Ca 0.2CrO 3, Y 0.8Ca 0.2Cr 0.8Co 0.2O 3, Y 0.8Ca 0.2Cr 0.9Ni 0.1O 3, deno-ted as YCC, YCCC and YCCN respectively, were synthesized by E

18、DTA e citric sol e gel method 32,33. Standard nitrates (AlfaAesar in stoichiometric percentage together with citric acid (AlfaAesar were dissolved into distilled water. EDTA powders (FisherScienti-c as complexant along with ammonia water (AlfaAesar was dissolved into the other set of distilled water

19、. The above solutions were blended together followed by adjusting pH to 8through ammonia water, then held at w 80 C and stirred until gelation. The gel was heated to 400 C to decompose nitrates and organic re-sidual. Resultant powders were calcined at different temperatures for several cycles with i

20、ntermediate ground to crystallize. After-ward, X-ray diffraction (XRD,PANalytical X pert PRO and Bruker AXS, Cu K a radiation test was conducted to examine purity of phase. Commercial software Jade 5was used to analyze XRD spectra. Powders were also pressed into pellet for scanning electron microsco

21、py (SEM,JEOL JSM-7600F observation.The calcined powders were pressed into pellet for DC conduc-tivity testing. Each piece of 0.6g powders was pressed into a pellet with 13mm diameter and w 1mm thickness under 220MPa. These pellets were then sintered at 1200e 1300 C for 4h. The porosity of each sampl

22、e was determined by Archimedes method (scalefrom Voyager Pro, model VP214CN. DC conductivity of the samples was measured using van der Pauw method with Au probes in ambient air at temperatures from 600to 850 C 34,35.To make anode slurry, these powders calcined at 1200 C were mixed with YSZ (TOSOH,8%

23、Y 2O 3stabilized and ground in ink vehicle (FuelCell Materials Co. The weight ratio of anode powders:YSZ:vehicle was 4:6:11.Anode slurry was screen printed onto both sides of YSZ electrolyte symmetrically. The circular YSZ electrolyte (NextechCo. used as cell support was 28mm in diameter and w 350m

24、m in thickness. The number of screen printing cycles depended on the designed electrode thickness. Typically, a single printing resulted in 5m m in thickness after sintering. The effect electrode area was 0.7cm 2at each side. The as-made symmetric cells were sintered at 1000 C for 2h in air. Pt mesh

25、 was bonded to both electrodes as current collector using Pt paste, followed by heating at 700 C.EIS testing was carried out on these symmetric cells in various H 2-containing atmospheres by Solartron 1287electrochemical interface and 1260impedance analyzer at open circuit condition over frequency r

26、ange from 0.1Hz to 99MHz. The AC signal applied was 20mV. H 2content in H 2/N2mixture was adjusted by mass ow controller (AlicatScienti c. H 2-containing gases were all mois-turized by passing through a water bubbler at room temperature before feeding to samples. The resultant spectra were deconvolu

27、ted using Z-view software.3. Results3.1. XRD patterns and SEM observationFig. 1shows the XRD spectra from the powder samples of pristine and doped YCCs. All patterns except the one for YCCC calcined at 1100 C are single phase, showing the orthorhombic perovskite structure (PDF#48-0474.For YCCC treat

28、ed at 1100 C, a foreign peak marked with a triangle at around 24.6 was detected and identi ed as the main peak of Ca 2Cr 2O 5(PDF#48-0791by Jade 5.0.It has been reported by K.J. Yoon, etc. that pure phase Ca and Co co-doped YCrO 3was obtained by glycine e nitrate method at 1200 C 18. And they also c

29、on rmed good chemical stability of such material in oxidizing and reducing atmosphere and good compatibility with YSZ. But in our study, it was observed that diffusion between YCCC pellet and YSZ felt holder occurred when sintered above 1200 C. The investigation of sintering behavior of Ca e Cr syst

30、em has been conducted in lots of literature 36,37. As studied in the CaO e Cr 2O 3phase system, liquid phase would appear when heating up to w 1022 C 38. Doping Ca into Cr-based perovskite is employed to improve the sinterability because the transient liquid phase 39. CaCrO 4, Ca 3(CrO4 2, Ca 5(CrO4

31、 3, etc., appear at different sintering phrases, and will eventually dissolve into the lattice when elevated temperature along with prolonged sintering time is used. What s more, Co is found to be able to complicate the transition of transient liquid phase further. Doping Co into (LaCa(CrCoO3facilit

32、ates the formation of liquid phase, resulting in an above 94%sinterability 40. Indeed, it was found herein that YCCC sample experienced a liquid sintering phrase in the middle of sintering process. Some area on the surface of YCCC pellet sintered at 1100 C was coated by a glass-like phase, which can

33、 be seen by bare eye and also observed by SEM as shown in Fig 2(a. Such phenomenon has not been observed on YCC or YCCN. The shrinkage simply calculated from the sizes of sample before and after sintering at 1400 C is 17%for YCCC, compared to 15%for YCC and 12%for YCCN. Based on the facts above, it

34、seems clear that Co doping facilitates the formation of Ca e Cr liquid phase, further improving the sinterability of yttrium chromites, even though Co itself somehow has not been signi cantly involved into this phase. EDS result for this glass-like phase shows the ratio of Y:Ca:Cr:Cois 0.1:1:1.1:0.0

35、3,in which the Ca:Crratio is consistent with the . u . a ( y t i s n e t n I 2(deg Fig. 1. XRD spectra of doped YCCs.496W. Li et al. /Journal of Power Sources 241(2013494e 501Fig. 2. Morphology of YCCC sintered at 1100 C (a,1200 C (b,YCC (cand YCCN (dsintered at 1200 C.secondary phase Ca 2Cr 2O 5ide

36、nti ed by XRD at 1100 C. The glass-like phase in YCCC disappeared as shown in Fig. 2(bwhen calcined at 1200 C. Accordingly, only a tiny peak at 24.6 is detected by the XRD for 1200 C calcined YCCC powder. Another consequence of Co facilitating sintering is that the particle size of YCCC is larger th

37、an those of YCC and YCCN sintered at the same temperature. As can be seen in Fig. 2, the grain size is 0.2e 0.5m m for YCC, 1e 1.5m m for YCCC and 0.2e 0.5m m for YCCN, respectively. 3.2. Electrical conductivitiesFig. 3a shows the electrical conductivities of YCCs in air. YCC, YCCC and YCCN pellets

38、were sintered in air at 1300 C, 1200 C, 1300 C, respectively, resulting in 93%,98%,and 86%relative den-sity. The data have been calibrated in terms of the porosity impact in a sphere sample 41. Both conductivities of YCCC and YCCN increased upon doping, but the increase was much more pronounced for

39、the former. The conductivity value reads 25, 44and 28S cm À1for YCC, YCCC and YCCN at 850 C, respectively. The value of YCCC is comparable with published results, while for YCC, it almost doubles 13. The corresponding Arrhenius curves are plotted in Fig. 3b. Activation energy abstracted from th

40、e slope of each curve for YCC, YCCC and YCCN is 0.18, 0.25and 0.19eV, respectively, which is in consistence with the reported data in Ref. 11.3.3. EIS results of YCCs in wet 5%H 2e N 2Fig. 4a shows the EIS for YCCN tested at 750 C in wet 5%H 2e N 2(5%H 2and 95%N 2. The solid lines are tting arcs fro

41、m Z-view program. Fig. 4b is the equivalent circuit used to t that spectrum, where L represents the overall inductor from leading wires and Solartron eternal circuit, R 0the total ohmic resistance composed of contributions from electrolyte, electrodes and leading wires. Con-stant phase element (CPEi

42、s adopted due to the frequency dispersion phenomenon in electrode process 42.As can be seen in Fig. 4a, the chosen equivalent circuit demon-strated a good representation of observed results. Four arcs in Fig. 4a imply at least four electrode processes are involved in H 2oxidation reaction at this gi

43、ven condition. But as temperature and atmosphere vary, each arc will evolve in its own way according mainly to its activation energy. By de nition,Y CPE ¼Q 0ðjw Þn ;(1where Q is the numerical value of admittance of Y CPE , j the imagi-nary unit and w the angular frequency, w ¼2p

44、f .From tting results, the characteristic frequency of each (R i CPE i can be derived from Eq. (243,f 0¼1n2p RQ 0(2The capacitance for (R i CPE i coupled with certain electrode process can be computed by Eq. (344,C ¼Q 0ð2w Þn À1sinp n2(3Table 1summarizes the resistance (R ,

45、capacitance (C , activation energy (E a and characteristic frequency (f 0 from EIS collected in wet 5%H 2at 850 C. In particular, Fig. 5displays EIS for YCCs in wet 5%H 2e N 2at 850 C tested at open circuit condition. The electrode thickness of all samples was w 15m m. The polarization resistance, i

46、.e. the difference between high and low frequency intercepts at real axis, is 235, 86and 51U cm 2for YCC, YCCC and YCCN, respec-tively, and meanwhile all these spectra possess roughly similar shape, indicating that after doping the performance of YCC has been enhanced signi cantly but the main elect

47、rode processes remain the same. At this temperature two arcs, arc II and IV as shown in Fig. 4a, are dominant, denoted as high frequency (HFarc and low frequency (LFarc separately.W. Li et al. /Journal of Power Sources 241(2013494e 501497a m c S ( bT (n l 1000/T (KFig. 3. a. Electrical conductivitie

48、s of YCCs in air. b. Arrhenius curves for YCCs tested in air.Fig. 6demonstrates the Arrhenius curves for HF arc derived from EIS of YCCs in 5%wet H 2. Fairly good linearity in each curve implies either the HF is related with a single electrode process alone or one single process is strongly predomin

49、ant among all the processes involved in HF range. Resistances from different samples can be ranked in order R YCC >R YCCC >R YCCN in the operating temperature range. The apparent activation energy (E a derived from Fig. 6isa'' Z -bZ'Fig. 4. a. Impedance of YCCN at 750 C in wet 5%H

50、2e N 2and the related tting arcs. b. Equivalent circuit used to t EIS.Table 1Summary of resistance (R , capacitance (C , activation energy (E a and characteristic frequency (f 0 for YCCs tested in wet 5%H 2e N 2at 850 C. In wet 5%H 2e N 2at 850 C R (U cm 2 C (FcmÀ2 E a (eVf o (HzHF arcLF arc182

51、 almost invariant, with values of 1.2, 1.3and 1.2eV for YCC, YCCC and YCCN, respectively. The data at 650 C seem to deviate from the trends for other temperatures for all three types of materials, indicating that the predominating processes for these materials may transit to different ones at lower

52、temperatures.Fig. 7shows the corresponding capacitance for HF arcs in wet 5%H 2calculated from Eq. (3. All of the capacitance for YCCs approx-imately keeps unchanged at different temperatures. This trend is in consistent with that reported for the double layer capacitance of Ni/YSZ anode by Primdahl

53、 and Mogensen 22.3.4. EIS results of YCCs in wet H 2e N 2with various H 2partial pressuresFig. 8displays the impedance of YCCN in wet H 2e N 2with various H 2partial pressures under open circuit condition at 850 C. The electrode thickness of this YCCN symmetrical cell is about 40m m. EIS testing was

54、 carried out in H 2partial pressure range from 10À1.5to 1atm by the increment of 100.25atm. Only one arc was observed in each spectrum in Fig. 8, except a very small distorted tail appearing at low frequency under very low P H 2. In contrast, the spectrum of cell with 15m m thick electrode rath

55、er consists of two arcs in Fig. 5where P H 2is 0.5atm. It has been reported that EIS spectra can be highly sensitive to microstructure and thickness of the SOFC electrodes. Jorgensen reported two EIS spectra different in magnitude and pro le were obtained from LSM/YSZcathodes in 4and 6m m thickness,

56、 respectively, due to variation in the micro-structure and composition 45. Brown addressed in detail the in-uence of microstructure on EIS using a range of Ni/YSZanode made out of different manufacturing conditions and raw materials 46. In particular, an active thickness of 10m m was demonstrated in

57、 their work based on the ionic conductivity of YSZ in the cermet structure. The electrode impedance changed signi cantly over this threshold. Besides these observations above, for YCCN electrode, another factor might also account for the variation of spectra upon change in electrode thickness. Compa

58、red to Ni/YSZor even to LSM/YSZ electrode, the electronic conductivity of YCCN e YSZ composite electrode is 2e 3orders lower. Poor conductivity of such electrode would certainly increase ohmic loss. But it may also lower the c (' ' Z -Z'(cm Fig. 5. Impedance of YCCs at open circuit condi

59、tion tested in wet 5%H 2e N 2at 850 C.498W. Li et al. /Journal of Power Sources 241(2013494e 501R /T (n l 1000/T (KFig. 6. Arrhenius curves for HF arc derived from EIS of YCCs in wet 5%H 2e N 2from 600to 850C. kinetics of electrode reactions involving electron, even though the resistivity of electron transportation will not be taken into account when dealing with kinetic reactions in electrodes with high elec-tronic