Isobe K.

Yamanishi T.

University of Toyama Repository

富山大学水素同位体科学研究センター研究報告29 ･ 30 : 51-54, 2010.Effects of Ceria Concentration in the Electrodeon Water I)ecomposition EfficiencyK. Isobe and T. YamanishiTritium Technology group, Japan Atomic Energy Agency, Tokai, Ibaraki, 31911195, JapanThe highly tritiated water is expected to be produced in the vacuum chamber and in ableeding blanket of fusion reactor system･ A ceramic electrolysis would be only a methodto recover tritium斤om such highly tritiated water. Aimlng at improvlng the waterdecomposition efficiency of this process, we have developed effective electrode containlngcerium oxide (ceria). In this study, the effect of ceria concentration in the electrode on theefficiency has been studied. The current density Increased with ceria concentration in theelectrode and reached the value or high density lOOm〟cm2 fわr 30% ceria-containing electrodeat l･5V This cu汀ent density is one order or magnitude higher than that of usual (Pt-YSZ)electrode.Keywords: tritium, highly tritiated water, water detritiation systemⅠ. IntroductionTritium and deuterium used as fuel in a fusionreactor present in its vacuum chamber andbreeding blanket during operation. Tritium ratio toother hydrogen isotopes in the vacuum chamber orbreeding blanket is expected to be from l% to 50%.Therefore, water presented in the vacuum chamberand bleeding blanket is highly tritiated and itsconcentration will reach 3･0×1018Bq･m-3 whentritium ratio is 50%･ Therefore it is one of the keyissues to recovery tritium from such highly tritiatedwater from the viewpolnt Of tritium inventorycontrol in the vacuum vessel as well as tritiumrecover as afuel. To reuse tritiated water asfuel,it should be reduced to hydrogen molecule whichwill be sent to the isotopes separation system infuel cycle system. In ITER, electrochemicalmethod uslng SOlid polymer electrolysIS is selectedfor Water Detritiation System (WDS). In thissystem, tritium concentration of processlng Wateris limited under l･1×1016Bq･m-3　due to theradiation damage of solid polymer. On the otherhand, a ceramic electrolysis can be used even inhigh beta-ray radiation doze because it is notdamaged by beta-ray from tritium. Therefore, theceramic electrolysis would be only a method torecover tritium from the highly tritiated water. Inthe development of Fuel Cleanup system fわr ITER,ceramic electrolysis systems f♭r processlng bothhydrocarbon and water had been developed [ 1日3】.For increase of efficiency on ceramic electrolysis,we had developed platinum and yttria stabilizedzirconia (Pt-YSZ) compound electrode. Forapplying these systems to process tritiated water,characteristic of components should be improvedand the efrlCiency of water reduction should beincreased. We have developed new two types orelectrode containing cerium oxide (ceria) toincrease 'the efrlCiency of water reduction l4]. Inthis study, We fbcused on the electrode containlngceria and the effect of ceria concentration in theelectrode on water decomposition efficiency wasexamined.51K. Isobe and T. YamanishiIl･ Principle of Ceramic Electrolysis MethodFigure 1 shows the schematic diagram ofpnncIPle of the ceramic electrolysis method whichis basically the same as the electrolysis of wateruslng a polymer electrolyte. In both cases, thereduction of water take place on the electrodesupplied with an electric potential. YSZ, which iswell used as an oxide ion conductor, is selected asa solid ceramic electrolyte because of its stableperfbmance. Electric charges needed fわr thereduction is supplied to the electrode on the YSZ ata high tempera山re (>873K) and water moleculesreaching to electrode are reduced on the electrode.Hydrogen molecules produced are released fromthe electrode to the environment. Oxygen ionsproduced are dissolved into YSZ and pemeatedthough it･ On the anode, Oxygen ions recombinewith each other to fbm oxygen molecules whichare released to environment･ Thus, Hydrogen andoxygen produced can be separated into thedifferent sides of electrolyte.O LQ lrge S⊂=> 乏ﾔр??E.ec.,.deA2ゝ血 唸耳耳痔｣｢罎ﾒ?啌ﾒ闔ｨﾟ? ▲1▲ ? 白ﾔ｢?Electrode 唸??停齏R龝ｸv?瀞鞴?罐｣?｢餮2??ー甲｢rmr, 02 -e- 劔 Fig･l Schematic diagram of principle or ceramicelectrolysIS methodTo improve the efficiency of the reductionreaction, many studies were carried out in the fieldofSolid Oxide Fuel Cell (SOFC). For example, thedevelopment of electrode has been performed todecrease over-Voltage needed fわr reaction and toincrease reaction polnt densities. Manufacturlngelectrodes with porous structure improves apparentmobility of water vapor and so on. In this study,we fbcused on the increase or reaction polntdensity on the electrode. Figure　2(a) showsschematic diagram around the reaction polnt. It isconsidered that electrode reaction takes place on52the three-phase b undary consistlng Of electrolyte,electrode and gasous phase. When electrode hasboth high oxygen ion conductivlty and electrononduct vity, reaction can occur on not only thethr e-pha e boundary but also the surface oflectrode, and the density of reaction polntS Willin rease as shown in Fig.2 (b). Ceria has not onlyhigh oxygen ion onductivlty like YSZ but alsoelect on conductivlty.　Therefbre electrodecontalnlng Ceria has been developed and showshigh p rformance in the field of SOFC.[4,5] Fromth  knowledge of SOFC, We tried to develop newelectrodes contalnlng Ceria to increase theefficie cy of wate  vapor reduction.(a)　H20　血H2(b)Electro yte.t2十二忘-p-h-ase b｡u nda"H2　_Hi:0H20 ⑳Electrolyte02-　　02-Fig.2　Schematic diagram orreaction point(a) Three-phase boundary model(b) Mixed conductivity electrodeIII. ExperimentalEl ctrodes conta ning different concentrationof Ce一ia were prepared f♭r this experiment. ThePt-Ceria astewith different mixing ratio (10%,20%, 30%) was coated on both sides ofYSZ disc(22mm in diameter, 1.5mmt). To collect electriccharge, Pt mesh w s coated on all electrodes. Tocompare the electro es containlng Ceria with usual(Pt-YSZ) one, the Pt-YSZ electrode in the samesize was also prepared. Composition of thisel ctrode is　90%　of Pt, 10%　of YSZ. Waterreduction experim nt was ca汀ied out at lO73K inEffects of Ceria Concentration in the Electrode on Water Decomposition EfficiencyArgon atmosphere under controlled water vaporc onc entrati on.IV. Results and DiscussionFigure 3 shows a plot or cu汀ent density orfour types of electrodes agalnSt applied voltage at2400ppm water vapor concentration. Theefficiency of decomposition can be expressed ascurrent, since only oxygen ions can diffuse thoughthe YSZ electrolyte.0　　0　　0　　0　　　0　　0　　　04208642[N∈｡＼VLLJ]^1!SuaPluGuLnO0　　03　　06　　09　1.2　15　1.8　　21Applled voltagelV]Fig･3 Cu汀ent density or electrodes agalnSt appliedvoltageCurrent density of all electrodes containing ce一ia ishigher than that ofPt-YSZ. This result shows Ceriain electrode enhances strongly waterdecomposition. 1n the ceria electrodes, the cu汀entdensity lnCreaSeS With ceria concentration in theelectrode and reaches high density ( 1 00mA/cm2) in30% ceria electrode at l･5V This cu汀ent denslty lSabout one order magnitude higher than that orPt-YSZ electrode. These results proved that ceriaconcentration in electrode strongly affects thewater decomposition efficiency and the usage ofsuch oxide containlng Can improve thedecomposition efrlCiency dramatica11y･ Thisimprovement of efficiency IS considered to be dueto not only the increase orreaction polntS but alsothe change or microstmcture of the electrode. Inthe previous paper [4], it was fわund that ceria inelectrode changed the surfacemicrostructure andmodified the electrode resistance･ To clarify theeffects of ceria concentration,　furtherinvestlgations, such as the obseⅣation of electrodeand the measurement of electrode resistance,would be required.V. Conclu onElec rodes contalnlng different concentrationof Ceria were prepared and water decompositioneffi iency of these electrodes was studied, Thecurrent density increased with ceria concentrationin the electrode and reached the value orlOOmA/cm2 for in 30% ceria-contalnlng electrodeat l･5V This cu汀ent density lS about one order ormagn血de higher than that of usual (PトYSZ)electrode･ These results proved that ceriaconcentration in electrode strongly affects thewater decomposition efrlCiency. To clarify theeffects of ceria concentration,　furtherinvestlgations, such as the obseⅣation of electrodeand the measurement of electrode resistance,would be required.References[1] S. Ko ishi, H. Yoshida,　H. Ohno, etal･, …Experiments on a ceramic electrolysIScell a d a palladium dimlSer at the tritiumsystems est assembly", Fusion Technol.8( 1 985)2042-2047.[2] S. Konishi, T. Maruyama, K. Okuno, et a1.,日Development of electrolytlC reactor fわrprocesslng Or gaseous tritiated compounds'',Fusion Eng. Des. 39-40(1998)1033-1039.[3] K. Isobe, H. Imaizumi, T. Hayashi, et a1.,"Demonstration  fuel cleanup systemco sistlng Or electrolytic reactor and tubulareservoir tank for fusion reactors". Fusion Sci.Tech. 41(2002)988-992.[4] K. Isobe., M. Uzawa and T. Ⅶmanishi,hDevelopment of Ceramic ElectrolysISMethod f♭r Processing High-Level Tritiatedwater,,, procee ings or　8th IAEA-TMFusion Power Plant Safety, 2006, ISBN53K. Isobe and T. Yamanishi92-0- 1 02007-4.[5] M. Watanabe, H. Uchida and M. Yoshida,  [6] H. Uchida, T. Osuga and M. Watanabe, "High"Effect of ionic conductivlty Of zirconia performance catalyzed-reaction layer forelectrolytes on the polarization behavior of medium temperature operating solid oxideceria-basedanodes in solid oxide fuel cells", J.　　　fuel cells", J.　Electrochem.　Soc.,Electrochem. Soc., 144(1997)1739-1743.　　　　　141(1999)1677-1682.54

Effects of ceria concentration in the electrode on water decomposition efficiency

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Effects of ceria concentration in the electrode on water decomposition efficiency

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