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Characterization of YSZ–YST composites for SOFC anodes

Hongpeng He *,Yingyi Huang,John M.V ohs,Raymond J.Gorte

Department of Chemical and Biomolecular Engineering,University of Pennsylvania,Philadelphia,P A 19104,USA

Abstract

Porous composites of Sr 0.88Y 0.08TiO 3àd (YST)and yttria-stabilized zirconia (YSZ)were formed by tape-casting methods and examined as potential anodes for Solid Oxide Fuel Cells (SOFC).Even after calcination to 1773K,the YSZ and YST crystallites remain as separate phases,as demonstrated by X-ray diffraction (XRD)and scanning electron microscopy (SEM)with EDX analysis.Furthermore,it was possible to fabricate anode-supported electrolytes by calcination of a bilayer tape formed by casting a thin YSZ tape over the green,YSZ–YST tape.Cells with anodes having 50-wt.%YST and 80-wt.%YST were tested and shown to yield good open-circuit voltages (OCV),although the power densities in H 2and CH 4between 973and 1173K were modest.Impedance spectra of the cells suggest that the conductivity of the YST–YSZ composites is insufficient for high performance.D 2004Elsevier B.V .All rights resercved.

Keywords:Solid oxide fuel cell;SrTiO 3;Zirconia;Anode;Hydrogen;Methane;SEM;Tape casting

1.Introduction

Most research in Solid Oxide Fuel Cells (SOFC)has used Ni-based anodes [1].While Ni anodes have demon-strated excellent performance in H 2and syngas,they have serious limitations for direct utilization of hydrocarbons due to the fact that Ni catalyzes the formation of carbon fibers [2–5].By using alternative conductors,such as Cu [6,7],doped ceria [8–10],and other conducting oxides [11–13],a number of groups have demonstrated that direct utilization of hydrocarbons is possible.Oxides that are electronically conductive under reducing conditions are particularly intriguing for anode applications.Unlike metallic anodes,oxides should be stable upon exposure to oxidizing conditions,an important feature for fuel cells that need to be shut down frequently.While oxides are certainly prone to sintering,one would expect oxide anodes to be more stable than metals to coarsening at high temperatures.Furthermore,oxides are generally more tolerant of impurities like sulfur than metals.

In general,the performance of ceramic anodes tends to be poor compared to metal-containing anodes.One reason for this is almost certainly that oxides have lower electronic conductivities than metals.However,another factor is the difficulty of forming an optimized anode–electrolyte interface with ceramic anodes.For example,with Ni-cermet anodes on yttria-stabilized zirconia (YSZ)electrolytes,mixtures of NiO and YSZ are fired to high temperatures so as to allow the YSZ in the anode to sinter with the YSZ electrolyte,forming a seamless,high-surface-area interface between the anode and the electro-lyte and also forming channels that allow ionic conduction into the electrode [14].Finally,it has not been possible to fabricate anode-supported electrolytes with ceramic ano-des,so that essentially all work with ceramic anodes has been with relatively thick electrolytes.

In this paper,we will discuss the fabrication of anode composites of YSZ and yttria-doped SrTiO 3(YST)by tape-casting methods.First,we will show that there is minimal reaction between these components,even at a calcination temperature of 1773K.Second,we will show that we can fabricate anode-supported,YSZ electrolytes with a wide range of YSZ–YST compositions.While the performance of cells made with these composites was relatively poor due to the low electronic conductivity that we were able to

0167-2738/$-see front matter D 2004Elsevier B.V .All rights resercved.doi:10.1016/j.ssi.2004.09.033

*Corresponding author.

E-mail address:hongpeng@https://www.sodocs.net/doc/8f10767807.html, (H.He).

Solid State Ionics 175(2004)171

–176

achieve,there are indications that one could use this approach to fabricate high-performance SOFC.

2.Experimental

The YST (Sr 0.88Y 0.08TiO 3àd )used in this study was synthesized by first mixing stoichiometric quantities of Sr(NO 3)2(99.0%,Alfa Aesar),Y(NO 3)3d 6H 2O (99.9%,Alfa Aesar)and TiO 2(99.9%,Alfa Aesar)in deionized water.After drying,the powder was heated overnight in air at 1273K,ground,and finally sintered in air at 1773K for 4h.X-ray diffraction (XRD)measurements demonstrated that the YST powder had the correct,perovskite structure.The YSZ was an 8-mol%Y 2O 3-stabilized zirconia (TZ-8T 8%Y)obtained from Tosoh.

Porous YST–YSZ composites with a range of composi-tions were prepared by tape casting,using methods similar to those used in making NiO–YSZ composites [15,16].In this paper,the composites will be designated by the weight percent YST in the composite (e.g.,The 50-wt.%YST–YSZ composite will be referred to as YST50.).After adding distilled water,a dispersant (Duramax 3005,Rohm &Haas),binders (HA12and B1000,Rohm &Haas),and pore formers (graphite and polymethyl methacrylate)to physical mixtures of YSZ and YST,the resulting slurry was cast into tapes that produced ceramic wafers,600A m in thickness.The porosity of all the ceramic wafers after calcination in air at 1773K was approximately 60%,as measured by water uptake measurements [15].

To prepare fuel cells from the YST–YSZ composites,a second tape,made from pure YSZ and without pore formers,was cast directly onto the green YST–YSZ tapes described above.The YSZ layer was targeted to have a thickness of 60A m.After calcining the two-layer tape in air to 1773K,a 50-

wt.%YSZ and LSM (La 0.8Sr 0.2MnO 3,Praxair Surface Technologies)mixture,with 10-wt.%graphite added as a pore former,was applied to the dense,YSZ side of the ceramic wafer as a paste,then calcined in air at 1523K to form the cathode.In most cells,the porous,YST–YSZ layer was impregnated with an ethanol solution of (NH 4)2Ce(NO 3)6and reduced in H 2at 723K to incorporate 10-wt.%ceria as a catalyst [17].Electronic contacts were formed using Pt mesh and Pt paste at the cathode and a Au wire with Au paste at the anode.

Each cell,having a cathode area of ca.0.35cm 2,was sealed onto 1.0-cm alumina tubes using Au paste and a ceramic adhesive (Aremco,Ultra-Temp 516).The entire cell was placed inside a furnace and heated to 973K at 2K/min.Dry H 2was fed to the anode at a flow rate of 100mL/min,while the cathode was simply left open to the air.In addition to measuring V –I curves,impedance spectra were obtained in the galvanostatic mode using a Gamry Instruments impe-dance spectrometer (Model EIS300),with a frequency range from 0.01Hz to 100kHz and using an amplitude of 3

mA/

Fig.1.XRD patterns of YSZ and the 50-wt.%YST–YSZ composite formed by tape-casting methods.The results indicate that there is no solid-state reaction between YSZ and YST,even after calcination to 1773K for 4

Fig.2.SEM micrographs of fracture of the composites made from (a)50-wt.%YST (YST50)and (b)80-wt.%YST (YST80).

H.He et al./Solid State Ionics 175(2004)171–176

172

cm2.Electrode microstructures were examined using a scanning electron microscope(SEM,JEOL JSM-6400) equipped with EDX for compositional analysis.

3.Results and discussion

Because of the possibility for solid-state reactions between YST and YSZ at high temperatures,the initial experiments were aimed at characterizing the porous composites formed by tape-casting mixtures of YST and YSZ.Fig.1shows the X-ray diffraction(XRD)pattern for pure YSZ and for YST50(50-wt.%YST–YSZ composite),prepared by tape casting,after firing to https://www.sodocs.net/doc/8f10767807.html,pared to pure YSZ,the composite exhibits new peaks at328,408,468,578,678,and7782h,all of which can be assigned to the perovskite phase of YST. No other phases,such as strontium zirconates,were observed.Obviously,this result could still be consistent with some migration of Ti or Y between phases,or with the formation of interfacial layers too thin to observe by XRD.

We also examined the YST50and YST80composites using SEM with EDX analysis,with the SEM micrographs of the fractured samples shown in Fig.2.The general appearance of the YST50composite,shown in Fig.2(a),is very similar to that of porous YSZ wafers made using similar methods and discussed in previous publications [7,15].The pores are interconnected,approximately5A m in size.A closer examination reveals that some of the crystallites are smooth,while others have small terraces. To determine the composition of the individual

crystallites, Fig.3.Results from EDX analysis for selected spots on the micrographs in Fig.2.

H.He et al./Solid State Ionics175(2004)171–176173

EDX measurements were performed at the spots indicated on the micrograph,with the spectra shown in Fig.3.Ignoring the Au peaks,which are due to metallization to make the sample conductive for SEM,the EDX results demonstrate that the b smooth Q crystallites are YSZ and the b terraced Q crystallites are YST.It is not clear whether traces of Ti have migrated into the YSZ phase.Fig.2(b),the micrograph of the YST80sample,shows somewhat larger,b terraced Q grains that can again be identified as YST,with smaller YSZ crystallites.

Fig.4shows V –I and power–density curves,in dry H 2at 1073K,for two fuel cells made with 60-A m YSZ electrolytes supported on 600-A m YST50composites.In one case,10-wt.%ceria was added to the porous YST50by wet impregnation.Both cells showed reasonable open-circuit voltages (OCV),greater than 1.15V at 973K with hydrogen,implying that the YSZ electrolyte was dense and has not formed significant cracks.(Obviously,the OCV in dry H 2also depends on the quality of the seals.)Furthermore,since any migration of Ti into the YSZ electrolyte would tend to make the YSZ electronically conductive,the excellent OCV measured on these cells demonstrates that solid-state reaction cannot have occurred on the length scale of the electrolyte.The fact that a dense YSZ electrolyte could be attached to the composite anode was somewhat surprising since some thermal-expansion mismatch should be expected between the YSZ and YST50layers over the very wide temperature range to which the bilayer was exposed in fabrication,300to 1773K,although an earlier report has also stated that La-doped SrTiO 3has a similar CTE as YSZ [12].The maximum power density improved significantly

with

Fig.3(continued ).

H.He et al./Solid State Ionics 175(2004)171–176

174

the addition of ceria,increasing from 12to 63mW/cm 2.In other work,ceria has been shown to improve the mixed ionic and electronic conductivity [18]and to improve the catalytic activity of the anode [19].The performance of the YST50cell with ceria was also measured at 973K and at 1173K in H 2,with the performance summarized in Table 1.At both temper-atures,the cell showed stable performance for 20h,with maximum power densities of 32mW/cm 2at 973K and 93mW/cm 2at 1173K.

To understand the factors limiting performance of the YST50cell with ceria,impedance spectra in Fig.5were measured with humidified H 2(3%H 2O)near OCV .At each temperature,the ohmic resistances,determined by the high-frequency intercept with the real axis,were much larger than the values that would be expected for a 60-A m YSZ electrolyte,ranging from N 5V cm 2at 973K to 2.3V cm 2at 1173K.By comparison,the ohmic resistance of cells from our laboratory with 60-A m electrolytes and the same cathodes,but having Cu-based anodes,was approximately 0.5V cm 2at 973K [20,21].The obvious conclusion is that the YST50composite does not have adequate electronic conductivity.It should be noted that we did not attempt a high-temperature reduction,which previous work has shown to be necessary for achieving high conductivity in YST [12].

Fig.6shows the performance curves for the YST50cell with ceria using humidified CH 4(3%H 2O)as the fuel.Power densities of 30and 80mW/cm 2were achieved at 1073and 1173K,values that are somewhat lower than that obtained for H 2.In agreement with the previous reports with methane on ceramic anodes,the performance of the cells was stable under these conditions.

Because 50-wt.%YST might not be sufficient to provide b percolation Q of the YST phase for electronic conductivity,a cell was also fabricated with an anode made from YST80and a 60-A m YSZ electrolyte,again with 10-wt.%ceria added to the YST80to enhance catalytic activity.The OCV and maximum power densities for the YST80cell are shown in Table 1for comparison to the YST50cell.The YST80cell again showed excellent OCV in H 2,demonstrating that a dense electrolyte had been obtained.However,the performance of this cell was actually slightly lower than that of the cell with YST50as the anode.We had expected that the higher fraction of YST in the anode would improve the electronic conduc-tivity;however,any increase in electronic conductivity appears to have been balanced by poorer ionic conductiv-ity or anode

structure.

Fig.4.Performance curves in dry H 2at 1073K for cells having a YST50anode,with and without the addition of 10-wt.%ceria.

Table 1

Summary of the performance data for the fuel cells made with YST50and YST80composite anodes using dry H 2and humidified CH 4as fuels Temperature (K)YST50YST80H 2

CH 4H 2

CH 497331(1.195)23(1.174)107362(1.165)35(1.01)48(1.167)20(1.057)1173

95(1.143)

78(1.05)

74(1.132)

48(1.083)

The power densities are given as mW/cm 2,while the values in parenthesis are the open-circuit

voltages.

Fig.5.Impedance spectra for the cell having a YST50anode with 10-wt.%ceria,using humidified H 2.The spectra were taken near OCV as a function of

temperature.

Fig.6.Performance curves in humidified CH 4for the cell having a YST50anode with 10-wt.%ceria as a function of temperature.

H.He et al./Solid State Ionics 175(2004)171–176175

4.Conclusions

The following new observations were made in this study: (1)Mixtures of yttria-stabilized zirconia(YSZ)and yttria-

doped SrTiO3(YST)can be heated to temperatures as high as1773K without undergoing substantial solid-state reactions.

(2)Composites of these two oxides can be made by tape-

casting methods.Porous structures composed of YSZ and YST crystallites can be fabricated with a wide range of compositions.

(3)Dense,thin YSZ electrolytes can be attached to

YST–YSZ composite anodes having YST composi-tions as high as at least80wt.%.The excellent open-circuit voltages(OCV)obtained on these cells demonstrate the formation of dense electrolytes without cracks,which in turn demonstrates that YSZ and YST have a reasonably good CTE match.

(4)The YST–YSZ composite anodes can be used for

direct utilization of CH4.

Acknowledgments

This work was supported by the Office of Naval Research.

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我爱家乡作文15篇(优秀版)

《我爱家乡作文》我爱家乡作文（1）：我爱家乡我的家乡在清远市三坑镇里的一个乡村叫上石村。那里的山虽然不高，但是，有很多花、草、树木，让人们能够吸收到清新的新鲜空气。那里的特产有：清远鸡、花生、地瓜我最喜欢吃地瓜啦！那里还有清清的小溪。夏天的时候人们就能够在小溪里游玩。那小溪还有小鱼呢！我喜欢小溪里的水，因为，那小溪水喝起来可清甜啦！那溪水好象山泉水一样。我爱家乡里的山，家乡里的水，还有那里的父老乡亲们！我爱家乡作文（2）：我的家乡是英德。她坐落在广东中北部，北江中下游。那里自然资源丰富，是广东的红茶之乡，盛产红茶、绿茶、苦丁茶。那里也是广东水泥之乡，又是广东石灰岩溶洞之乡，有旅游胜地：宝晶洞宫、英西峰林。英德还是历史名城，北江岸边，南山倒影，摩崖石刻，令人神往。我的家乡多数是客家人，经常讲客家话。我只会一点点客家话。那里特产众多，有可口的西牛笋干、柔嫩的九龙豆腐，还有奇异的广东英石、香甜的擂茶粥。这些小吃，可好吃了！我爱我的家乡，因为那就是我最好的家乡。我爱家乡作文（3）：我的家乡在广东省清远市，是一个十分美丽的地方，是中国优秀的旅游城市，也是闻名的温泉之乡、漂流之乡、龙舟之乡。

我的家乡风景秀丽，美丽的北江将整个城市划分为新城和旧城。我最喜欢去的地方是北江河畔，那里环境优美，是全市人民休息和娱乐的场所。每一天晚上都有很多人去北江河畔散步、跳舞但是，我最喜欢白天去北江河的沙滩上玩，那里留下了我们全家人的笑声和完美的回忆。我的家乡物产丰富，有味道鲜美的清远鸡、乌鬃鹅、骆坑笋、北江河鲜等。我爱我的家乡！欢迎大家来我的家乡旅游和购物！我爱家乡作文（4）：我的家乡在清远，清远是一座美丽的城市，这几年变化可大了，以前，北江两岸只有沙子和旧房子，此刻建成了一座座高楼大厦。以前没有江滨公园，那里只是人们倒垃圾的地方，此刻有了江滨公园，那里绿树成荫、鸟语花香，空气清新，人们有了散步、休息、做操的好地方。清远有很多特产，有清远鸡，乌鬃鹅，骆坑笋。。。。。。清远有很多好玩的地方，有牛鱼嘴、地下河、温泉，漂流。。。。。。我的家乡真美丽，我爱我的家乡。我爱家乡作文（5）：我爱无边无际的草原，我爱连绵起伏的山峰，我爱波涛汹涌的大海你要问我最爱什么，我爱我的家乡。我的家乡在鲁西南，那是一片无边无际的平原，盛产粮棉，村庄密布，绿树如织。我的家乡是鲁西南平原上最耀眼的星星，黄河像一条金丝带从她的西北穿过，京杭大运河像一条碧纱横挽南北，烟波浩淼的东平湖像一颗宝珠镶嵌在她的头冠上。我的家乡在水泊梁山，《水浒传》的故事就发生在那里。这块土地英雄辈出，名扬海外。我喜欢家乡人豪爽的性格，更喜欢家乡的自然风景。春天，小草探出了脑袋，花儿绽开了笑脸，柳树抽出了嫩绿的枝条随风舞蹈，展现她那婆娑的舞姿。燕子从南方飞往那里，在那里筑巢、安家。春雨滴滴答答地下着，为树木洗澡、为花朵梳妆。春雨过后，竹笋快速地生长，长出了嫩绿的新芽、露出了调皮的脸庞夏天，小草更绿了、花儿更艳了、大山披上绿装。人们在大树下乘凉、说笑。一场大雨过后，新鲜的空气沁人心脾。天空的白云变化无穷，一会儿似龙、一会儿似虎秋天，树木换上了金装，叶子从树上落了下来，像一只只翩翩起舞的蝴蝶，飞入大地的怀抱。田地里，农民们正忙着收割稻子、高粱、豆子等农作物，他们的脸上个个露出了丰收的喜悦。

自我管理演讲稿英语翻译

尊敬的领导，老师，亲爱的同学们，大家好！我是5班的梁浩东。今天早上我坐车来学校的路上，我仔细观察了路上形形色色的人，有开着小车衣着精致的叔叔阿姨，有市场带着倦容的卖各种早点的阿姨，还有偶尔穿梭于人群中衣衫褴褛的乞丐。于是我问自己，十几年后我会成为怎样的自己，想成为社会成功人士还是碌碌无为的人呢，答案肯定是前者。那么十几年后我怎样才能如愿以偿呢，成为一个受人尊重，有价值的人呢？正如我今天演讲的题目是：自主管理。大家都知道爱玩是我们孩子的天性，学习也是我们的责任和义务。要怎样处理好这些矛盾，提高自主管理呢？首先，我们要有小主人翁思想，自己做自己的主人，要认识到我们学习，生活这一切都是我们自己走自己的人生路，并不是为了报答父母，更不是为了敷衍老师。我认为自主管理又可以理解为自我管理，在学习和生活中无处不在，比如通过老师，小组长来管理约束行为和同学们对自身行为的管理都属于自我管理。比如我们到一个旅游景点，看到一块大石头，有的同学特别兴奋，会想在上面刻上：某某某到此一游话。这时你就需要自我管理，你需要提醒自己，这样做会破坏景点，而且是一种素质低下的表现。你设想一下，如果别人家小孩去你家墙上乱涂乱画，你是何种感受。同样我们把自主管理放到学习上，在我们想偷懒，想逃避，想放弃的时候，我们可以通过自主管理来避免这些，通过他人或者自己的力量来完成。例如我会制定作息时间计划表，里面包括学习，运动，玩耍等内容的完成时间。那些学校学习尖子，他们学习好是智商高于我们吗，其实不然，在我所了解的哪些优秀的学霸传授经验里，就提到要能够自我管理，规范好学习时间的分分秒秒，只有辛勤的付出，才能取得优异成绩。在现实生活中，无数成功人士告诉我们自主管理的重要性。十几年后我想成为一位优秀的，为国家多做贡献的人。亲爱的同学们，你们们？让我们从现在开始重视和执行自主管理，十几年后成为那个你想成为的人。谢谢大家！

关于工作的英语演讲稿

关于工作的英语演讲稿【篇一：关于工作的英语演讲稿】关于工作的英语演讲稿 different people have various ambitions. some want to be engineers or doctors in the future. some want to be scientists or businessmen. still some wish to be teachers or lawers when they grow up in the days to come. unlike other people, i prefer to be a farmer. however, it is not easy to be a farmer for iwill be looked upon by others. anyway,what i am trying to do is to make great contributions to agriculture. it is well known that farming is the basic of the country. above all, farming is not only a challenge but also a good opportunity for the young. we can also make a big profit by growing vegetables and food in a scientific way. besides we can apply what we have learned in school to farming. thus our countryside will become more and more properous. i believe that any man with knowledge can do whatever they can so long as this job can meet his or her interest. all the working position can provide him with a good chance to become a talent. 【篇二：关于责任感的英语演讲稿】 im grateful that ive been given this opportunity to stand here as a spokesman. facing all of you on the stage, i have the exciting feeling of participating in this speech competition. the topic today is what we cannot afford to lose. if you ask me this question, i must tell you that i think the answer is a word---- responsibility. in my elementary years, there was a little girl in the class who worked very hard, however she could never do satisfactorily in her lessons. the teacher asked me to help her, and it was obvious that she expected a lot from me. but as a young boy, i was so restless and thoughtless, i always tried to get more time to play and enjoy myself. so she was always slighted over by me. one day before the final exam, she came up to me and said, could you please explain this to me? i can not understand it. i

关于时间管理的英语演讲

Dear teacher and colleagues: my topic is on “spare time”. It is a huge blessing that we can work 996. Jack Ma said at an Ali's internal communication activity, That means we should work at 9am to 9pm, 6 days a week .I question the entire premise of this piece. but I'm always interested in hearing what successful and especially rich people come up with time .So I finally found out Jack Ma also had said :”i f you don’t put out more time and energy than others ,how can you achieve the success you want? If you do not do 996 when you are young ,when will you ?”I quite agree with the idea that young people should fight for success .But there are a lot of survival activities to do in a day ,I want to focus on how much time they take from us and what can we do with the rest of the time. As all we known ,There are 168 hours in a week .We sleep roughly seven-and-a-half and eight hours a day .so around 56 hours a week . maybe it is slightly different for someone . We do our personal things like eating and bathing and maybe looking after kids -about three hours a day .so around 21 hours a week .And if you are working a full time job ,so 40 hours a week , Oh! Maybe it is impossible for us at