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 Efficiency
K. Isobe and T. Yamanishi
Tritium Technology group, Japan Atomic Energy Agency, Tokai, Ibaraki, 31911195, JapanThe highly tritiated water is expected to be produced in the vacuum chamber and in a

bleeding blanket of fusion reactor system･ A ceramic electrolysis would be only a method
to recover tritium斤om such highly tritiated water. Aimlng at improvlng the water

decomposition efficiency of this process, we have developed effective electrode containlng
cerium 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 system

hand, a ceramic electrolysis can be used even inⅠ. Introduction

high beta‑ray radiation doze because it is not

Tritium and deuterium used as fuel in a fusion
damaged by beta‑ray from tritium. Therefore, thereactor present in its vacuum chamber andceramic electrolysis would be only a method tobreeding blanket during operation. Tritium ratio torecover tritium from the highly tritiated water. In
other hydrogen isotopes in the vacuum chamber or

breeding blanket is expected to be from l% to 50%.

the development of Fuel Cleanup system fわr ITER,
ceramic electrolysis systems f♭r processlng both

Therefore, water presented in the vacuum chamberhydrocarbon and water had been developed [ 1日3】.

and bleeding blanket is highly tritiated and its

For increase of efficiency on ceramic electrolysis,
concentration will reach 3･0×1018Bq･m‑3 when

tritium ratio is 50%･ Therefore it is one of the key

we had developed platinum and yttria stabilized

zirconia (Pt‑YSZ) compound electrode. For
issues to recovery tritium from such highly tritiatedapplying these systems to process tritiated water,water from the viewpolnt Of tritium inventory
characteristic of components should be improved
control in the vacuum vessel as well as tritium

recover as afuel. To reuse tritiated water asfuel,and the efrlCiency of water reduction should beincreased. We have developed new two types orit should be reduced to hydrogen molecule whichelectrode containing cerium oxide (ceria) towill be sent to the isotopes separation system infuel cycle system. In ITER, electrochemical

increase 'the efrlCiency of water reduction l4]. In
this study, We fbcused on the electrode containlng

method uslng SOlid polymer electrolysIS is selected

for Water Detritiation System (WDS). In thisceria and the effect of ceria concentration in theelectrode on water decomposition efficiency wassystem, tritium concentration of processlng Water

examined.
is limited under l･1×1016Bq･m‑3

due to theradiation damage of solid polymer. On the other51

K. Isobe and T. Yamanishi

Il･ Principle of Ceramic Electrolysis Method

the three‑phase boundary consistlng Of electrolyte,electrode and gasous phase. When electrode has
Figure 1 shows the schematic diagram ofboth high oxygen ion conductivlty and electron

pnncIPle of the ceramic electrolysis method which

conductivity, reaction can occur on not only theis basically the same as the electrolysis of water

three‑phase boundary but also the surface ofuslng a polymer electrolyte. In both cases, theelectrode, and the density of reaction polntS Will

reduction of water take place on the electrodeincrease as shown in Fig.2 (b). Ceria has not only

supplied with an electric potential. YSZ, which ishigh oxygen ion conductivlty like YSZ but also

well used as an oxide ion conductor, is selected aselectrona solid ceramic electrolyte because of its stablecontalnlng Ceria has been developed and shows

perfbmance. Electric charges needed fわr thehigh performance in the field of SOFC.[4,5] From

reduction is supplied to the electrode on the YSZ at

the knowledge of SOFC, We tried to develop newa high tempera山re (>873K) and water molecules

electrodes contalnlng Ceria to increase thereaching to electrode are reduced on the electrode.

efficiency of water vapor reduction.

conductivlty.

Therefbre

electrodeHydrogen molecules produced are released fromthe electrode to the environment. Oxygen ions(a)

H20

血H2

produced are dissolved into YSZ and pemeatedthough it･ On the anode, Oxygen ions recombinewith each other to fbm oxygen molecules which

Electrolyte.t2十二忘‑p‑h‑ase b｡u nda"

are released to environment･ Thus, Hydrogen andoxygen produced can be separated into the(b)different sides of electrolyte.

H2

̲Hi:0

H20 ⑳O

LQlrgeS⊂=>乏ﾔр

E.ec.,.deA2ゝ血 唸耳耳痔｣｢罎ﾒ剪啌ﾒ闔ｨﾟ

▲1▲

白ﾔ｢Electrolyte02‑Electrode 唸3ﾔ停齏R龝ｸvー甲｢rmr, 02
謫ﾒ鞴

Fig.2粤｣｣02‑

Schematic diagram orreaction point｣｢餮2(a) Three‑phase boundary model‑e‑ 劔(b) Mixed conductivity electrodeFig･l Schematic diagram of principle or ceramicelectrolysIS method

III. Experimental

To improve the efficiency of the reduction
reaction, many studies were carried out in the field

Electrodes containing different concentrationofSolid Oxide Fuel Cell (SOFC). For example, theof Ce一ia were prepared f♭r this experiment. The

development of electrode has been performed to

Pt‑Ceria pastewith different mixing ratio (10%,decrease over‑Voltage needed fわr reaction and to20%, 30%) was coated on both sides ofYSZ discincrease reaction polnt densities. Manufacturlng(22mm in diameter, 1.5mmt). To collect electric

electrodes with porous structure improves apparent

charge, Pt mesh was coated on all electrodes. To

mobility of water vapor and so on. In this study,

compare the electrodes containlng Ceria with usual

we fbcused on the increase or reaction polnt(Pt‑YSZ) one, the Pt‑YSZ electrode in the samedensity on the electrode. Figure

size was also prepared. Composition of this2(a) shows

schematic diagram around the reaction polnt. It iselectrode isconsidered that electrode reaction takes place on

reduction experiment was ca汀ied out at lO73K in

5290%of Pt, 10%of YSZ. Water

Effects of Ceria Concentration in the Electrode on Water Decomposition Efficiency
Argon atmosphere under controlled water vapor

effects

c onc entrati on.

investlgations, such as the obseⅣation of electrodeofceria

concentration,

further

and the measurement of electrode resistance,

IV. Results and Discussion

would be required.

Figure 3 shows a plot or cu汀ent density orV. Conclusion

four types of electrodes agalnSt applied voltage at2400ppm water vapor concentration. The

Electrodes contalnlng different concentration

efficiency of decomposition can be expressed asof Ceria were prepared and water decomposition

current, since only oxygen ions can diffuse though

efficiency of these electrodes was studied, The

the YSZ electrolyte.

current density increased with ceria concentrationin the electrode and reached the value or

lOOmA/cm2 for in 30% ceria‑contalnlng electrode4208642 0 0 0 0 0 0 0at l･5V This cu汀ent density lS about one order or[N∈｡＼VL J]^1!SuaPluGuLnO

magn血de higher than that of usual (PトYSZ)

electrode･ These results proved that ceria

concentration in electrode strongly affects thewater decomposition efrlCiency. To clarify theeffects

ofceria

concentration,

further

investlgations, such as the obseⅣation of electrode

00306

09

1.2

15

1.8

21

Applled voltagelV]

and the measurement of electrode resistance,
would be required.

Fig･3 Cu汀ent density or electrodes agalnSt applied

voltage

Current density of all electrodes containing ce一ia isReferences[1]

S.

Konishi,H.

Yoshida,

H.

Ohno,

et

higher than that ofPt‑YSZ. This result shows Ceria

al･, …Experiments on a ceramic electrolysISin electrode enhances strongly watercell and a palladium dimlSer at the tritium

decomposition. 1n the ceria electrodes, the cu汀ent

systems test assembly", Fusion Technol.

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electrode and reaches high density ( 1 00mA/cm2) in
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Pt‑YSZ electrode. These results proved that ceria

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concentration in electrode strongly affects thewater decomposition efficiency and the usage of[3] K. Isobe, H. Imaizumi, T. Hayashi, et a1.,
"Demonstration of fuel cleanup systemsuch oxide containlng Can improve theconsistlng Or electrolytic reactor and tubular

decomposition efrlCiency dramatica11y･ This

reservoir tank for fusion reactors". Fusion Sci.

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Tech. 41(2002)988‑992.

to not only the increase orreaction polntS but also[4] K. Isobe., M. Uzawa and T. Ⅶmanishi,

the change or microstmcture of the electrode. In

hDevelopment of Ceramic ElectrolysIS

the previous paper [4], it was fわund that ceria inMethod f♭r Processing High‑Level Tritiatedelectrode changed the surfacemicrostructure andwater,,, proceedings ormodified the electrode resistance･ To clarify theFusion Power Plant Safety, 2006, ISBN

8th IAEA‑TM53K. Isobe and T. Yamanishi
92‑0‑ 1 02007‑4.

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