- Email: [email protected]
Solid oxide electrolysis cell for decomposition of tritiated water
0360-3199/86 $3.00 + 0.00 Pergamon Journals Ltd. International Association for Hydrogen Energy.
Int. J. Hydrogen Energy, Vol. 11, No. 8, pp. 507-512, 1986. Printed in Great Britain.
SOLID OXIDE ELECTROLYSIS CELL FOR DECOMPOSITION OF TRITIATED WATER S. KONISHI, H. OHNO*, H. YOSHIDA, H. KATSUTA* and Y. NARUSE Department of Thermonuclear Fusion Research, *Department of Fuels and Materials Research, Japan Atomic Energy Research Institute, Tokai, Ibaraki, 319-11 Japan (Received for publication 10 January 1986) Abstract--The decomposition of tritiated water vapor by means of solid oxide electrolysis cells has been proposed for the application to the D-T fusion reactor system. This method is essentially free from problems such as large tritium inventory, radiation damage, and generation of solid waste, so it is expected to be a promising one. Electrolysis of water vapor in an argon carrier was performed using a tube-type stabilized zirconia cell with porous platinum electrodes over the temperature range 500-950°C. High conversion ratios from water to hydrogen, of up to 99.9%, were achieved. The characteristics of the cell were deduced from the Nernst equation and the conversion ratios expressed as a function of the/R-free voltage. Experimental results agreed with the equation. The isotope effect in electrolysis is also discussed and experiments with heavy water were carried out. The obtained separation factor was slightly higher than the theoretical value.
NOMENCLATURE a H 2, a H 2 0 , a o 2
E Eo
F
£,f AG, A G ° l,P
PH2, PH20, Po2
R R T X O,/
Activities of H2, H 2 0 and 02 required cell voltage for the electrolysis of water required voltage for the electrolysis of water at standard state and temperature T Faraday constant molar flow rates of water vapor at the inlet and outlet of the cell, respectively Gibbs free energy of formation for water or heavy water (standard state) current through the cell in operation and corresponding to the molar flow rate of water into the cell partial pressures of H2, H20 and O2 gas constant [J tool -1K -1] resistance of the cell temperature [K] conversion ratio from water to hydrogen separation factor between protium(H) and deuterium(UD). INTRODUCTION
In deuterium-tritium ( D - T ) fusion reactor systems, tritiated water forms in various processes such as breeding blankets, fuel clean-up systems for the reprocessing of plasma exhaust and recovery processes for tritium leakage from the primary enclosure. It is desirable to decompose this tritiated water and to recover tritium as hydrogen gas for fueling the reactor core. Although some techniques such as activated metal getters [1], conventional wet electrolysis cells and solid polymer electrolyte(SPE) cells can be applied, they suffer the 507
disadvantages of periodic replacement and generation of solid waste, large tritium inventory and radiation damage by tritium [2], respectively. Decomposition of water vapor using solid oxide electrolysis cells has been proposed for this purpose [3]. Such a cell would have a small tritium inventory, high tritium compatibility and would operate continuously in the gas system. This process should produce little radioactive waste because the ceramic electrolyte, through which only oxygen ions can pass and hydrogen permeability is expected to be very small, will not allow contamination of the exhaust oxygen by any significant amounts of tritium if the cell is completely gas tight. This concept has been proposed as a technique for hydrogen production [4, 5] and is intended to be operated at high temperatures around 1000°C. For application to the fusion reactor fuel systems, safe and reliable operation of the cell is important, while efficiency with regard to electrical energy is not essential. In order to design the cell and to determine optimum operating conditions, detailed characteristics of the cell with hydrogen isotopes at temperatures lower than the case for hydrogen production were required. In this paper, characteristics of the solid oxide electrolysis cell are discussed with regard to electrochemical considerations and experimental data. The isotope effect in the electrolysis reaction is also estimated and measured experimentally with protium (light water) and deuterium (heavy water). E Q U A T I O N S F O R THE CELL CHARACTERISTICS An electrolysis cell is shown schematically in Fig. 1. The cell is composed of two electrodes with an oxygen ionic conductor between them. At the cathode, reduction of water vapor: H 2 0 ~ H 2 + 0 2 - proceeds,
508
S. KONISHL H. OHNO, H. YOSHIDA, H. KATSUTA AND Y. NARUSE In terms of currents I = 2F(f0 - f ) , I,i = 2F)~, (4) can also be written
Inlet
gO2= l I
Outlet
E - I R = Eo + (RT/2F) l n ( I / I o - I),
H20 : fo__.. H2 :fo-f
PH20 : Po ...... "v" H20 H2
and the conversion ratio of water to hydrogen, x = ()Co - f ) / f o ,
PH2 : P o l o f
E - I R = E ° + (RT/2F) ln(x/1 - x).
PH20: PO+
The value E - I R is the/R-free voltage of the cell which is related to the composition of the product. It should be noted that the conversion ratio x depends on the/R-free voltage, E - I R , and is not affected by the pressure or flow rate of the reactant. The relationships between conversion ratio x and /R-free voltage, are calculated from (6) and summarized in Fig. 2. As seen in the figure, the conversion ratio rises to 99.9% at about 1.3 V and approaches unity at higher voltage. The values of E ° at each temperature corresponds to the points x = 0.5. Because of the endothermic nature of the reaction,/R-free voltage decreases with increasing temperature. Due to the temperature dependence of the logarithmic term in (6), however, this effect decreases with the value of x. For the various isotopes of hydrogen, only the constant term E ° in (6) is different because of the difference of Gibbs free energy of formation AG °. In the case of electrolysis of HzO and D20, the difference in voltage for the same conversion ratio x is calculated as follows.
Fig. 1. A model of the solid oxide electrolysis cell.
while the anode reaction is formation of oxygen gas: 02-~½02. These reactions are summed to H20 ---*Ha + ½02 and the Gibbs free energy change for the reaction, AG, is, a0.5 aHz oz
AG = AG ° + R T l n ~
(1)
aH20
where AG ° is the standard Gibbs free energy change (per mole) for the reaction HzO--*Hz + ½Oz at temperature T and an2, a02 and all20 are the activities of H2, 02 and H 2 0 in the cell, respectively. This relation can also be written E = E ° + (RT/2F) ln(aH2 a o0.5 JaH2o).
(2)
This equation is equivalent to the Nernst equation for the e.m.f. (electromotive force) of a fuel cell using H 2 and 02, where E ° is the standard e.m.f, of the reaction H2 + ½02---~ H20. Activities of reactants and products are expressed as partial pressures in the cell because the reaction proceeds in the gas phase regardless of the electrolyte. For a cell being operated with current I and apparent resistance R, the ohmic(IR) loss is related to the difference in electrochemical potential of oxygen between anode and cathode. Then (2) is rewritten E = E ° + (RT/2F)ln(Pn2 P ~ / P r h o )
(5)
+ IR
(3)
where Po2 is the partial pressure of 02 at the anode. In the model cell shown in Fig. 1, water vapor of molar flow rate f0 and pressure P0 is reduced to hydrogen gas of flow rate)C0 - f and partial pressure Po(fo - f ) / f o in the cathode compartment. The partial pressure of oxygen at the anode, Po2, is assumed to be unity. The number of molecules does not change in the reaction at the cathode. Gaseous reactants and products are assumed to be mixed well so that almost all the molecules are carried to the reaction point on the cathode surface. In this condition, the gases in the cell are related to the applied voltage on the cathode, and the composition of the gas at the outlet of the cell in the steady state is expressed by (3). Using the molar flow rates, (3) can be written E - I R = E ° + (RT/2F)In(f0 - f / f ) .
(4)
ED -- EH = ( AGo D20 -- AG° n2o)/2F
= 38.3 - 2.49 × 10 -2 T
(6)
(7)
(mV)
where AG~2 o and AG~2 o are the Gibbs free energy of formation for D20 and H20, respectively. The isotope effect in the electrolysis of water is known to be very large compared with other processes. However, as seen from (7), the effect decreases with the temperature and is not so significant at high temperatures. It should be noted that the difference AG~2 o - A G ~ 2 o also expresses the equilibrium constant for the exchange reaction D2 + H 2 0 ~ H 2 + D 2 0 as, AG~2o - AG~I2O = -RTln(Pr~z. P H 2 o ) / ( P D 2 o .
PHz).
(8)
This relationship indicates that the isotope effect in electrolysis is equivalent to that in the isotope exchange reaction. The separation factor o~ is defined as the ratio of isotopes in the vapor phase (H20, HDO, D20) to those in the hydrogen gas phase (H2, HD, D2) in the product stream by o~= (Hgas/Dgas)/(H,,p/Ovap) ( P n z + ½PHD) (PD20 + ½PHDo)
-- ( e o z + ½PHD) (PHzo + ½PHDo)
(9)
This factor can be related to the (7) and is expected to decrease with temperature.
DECOMPOSITION OF TRITIATED WATER
"T -
,oo/////// o.,
measured by this device. The cell was operated in the temperature range 500-950°C. Temperature was measured with a Pt vs 13%Rh-Pt thermocouple, The isotope effect in the electrolysis reaction was measured with D20 or mixtures of D20 and H20. The quantitative analysis of protium(H) and deuterium(D) in the product stream was made using a Mass Spectrometer and Multi-Ion Selector (ANELVA TE-150 and MIS-200).
Temperoture 900800_~/////~,,,///// zoo-_/j_y//ll
o o
o
. . . . . .
/ ////I/1.; / / / / t /" 50o
;
509
RESULTS AND DISCUSSIONS 1.2
IR-free
Vollege
1.3
(V)
Fig. 2. Relationships between/R-free voltage and conversion ratio of water to hydrogen obtained numerically.
EXPERIMENTAL Zirconia cells stabilized with calcia, yttria and other rare earth oxides were prepared as electrolytes. In this study, most of the experiments were carried out with zirconia stabilized with 8 mol% yttria, this being the most proven and conductive material. The electrolytes were frabricated into the shape of a 13 mm o.d., 9 mm i.d. test tube. On the inner and the outer surface, porous electrodes of about 20 cm 2 were formed by baking platinum paste which had been doped onto the surfaces. The arrangement of the experimental apparatus is shown in Fig. 3. The electrolysis cell was mounted in a quartz tube and heated by an electric furnace. Reactants and products were carried by an argon stream with a flow rate regulated to 10-300 cm 3 min-Z. Concentration of water vapor, from which the conversion ratio was estimated, was measured with hygrometers (Panametrics-700) at the inlet and the outlet of the cell. A potentiostat (Hokuto Denko HA-301), which supplies stable cell voltages with errors less than 0.1%, was used as a power source. The electrolysis current was also O~t
Variation of the cell resistance with temperature is shown in Fig. 4. Conductivity can be written in an Arrhenius form, though in the high temperature region, a residual resistance of about 2 ~ was observed. This seemed to be caused by contact resistance at the interface between electrodes and electrolyte. Some examples of relationships between voltages and current during electrolysis are shown in Figs 5a, b obtained at 859°C and 506°C, respectively. Closed circles represent values for the cell voltage and open circles the/R-free voltage, ( E - I R ) , estimated from cell voltage, resistance and current. Figures 6a, b show the relationships between cell voltage and the conversion ratio from water to hydrogen, compared with the prediction from (6) as indicated by the solid lines. The highest values of the conversion ratio x were about 95% for lower temperatures and up to 99.95% for high temperatures. The deviation of conversion ratio from unity is caused by residual water in the tubing between the cell and the outlet hygrometer. This was calculated as water which failed to be decomposed by electrolysis. As seen in Fig. 6a, the experimental results agreed well with the theoretical values calculated from (6) at high temperatures. The discrepancy observed in Fig. 6b indicates the existence of an overpotential neglected in (6).
Ternperoture 900 800 700 600
(°C)
500
400
103
H2
H~ (
Slobilized ; Pf - elecl 310
Ele Fur
O
Gos ,
F~, F2 : Moss Flow Meter H~, He : Hygrorneler Fig. 3. Experimental set-up of electrolysis cell.
0.8
I
1.0
,
I
1.2
,
1
1.4
,
]
1.6
Reoiprocol Temperofure IOOO/T (K-I) Fig. 4. Apparent resistance of the electrolysis cell shown against the reciprocal temperature.
510
S. KONISHI, H. OHNO, H. YOSHIDA, H. KATSUTA AND Y. NARUSE 250
'
ioo o
859 °C
E200
• 6
'
0
•
o
O
1.2
o
O
o
~ is0 ~g
LLI
o o
. ~ 100
o
•
o
• Cell Voltage
•
o oe
•
f.0
0.5
1.0
IR-free Voltage
o
'7
I 20
1/5
25
Voltage (v)
2O
r
( o o o o
E f5
0
0
0
I
f000
Fig. 7. Temperature dependence of the value of EV2, i.e. the/R-free voltages corresponding to x = 0.5, which are the experimental values of E ~ = AG°/2F. The solid line shows the theoretical value.
•
• •
OT O •
015
J
Temperature (°C)
•
o
0
I
800
•
O
o
J
• •
g
o
I
600
•1
oo
509°C
l
080o
(0}
'0
• Cell Voltage o IR-free Voltage 'l~
~ l0
6O
~~
Voltage (v) 40
(b) Fig. 5. Voltage-current characteristics of the cell during electrolysis at (a) 859°C and (b) 509°C. The open circles shows the /R-free voltages and the closed circles show the cell voltage. This fact suggests that the reaction on the cathode surface is very fast and is reversible, perhaps due to the catalytic activity of the platinum electrodes and/or the very high temperatures. Figure 7 shows the values of E1/2, which are obtained from t h e / R - f r e e voltages for x = 1/2, compared to the theoretical value, E °. Though the data are scattered, the deviation decreases with
temperature.
I
2O
0 0
0
-----"0 . . . . . .
0
o
--
.~
~_~.~
O
0 -20
I
I
I
700
800
900
Temperature
(°C)
Fig. 8. Differences in the /R-free voltage for electrolysis of heavy water and light water. The solid line shows the theoretical value. fO I
1.0
I
~
I 0 ~
o
l
I
509°C
o 08
0.8
-io
5-
° o
o
0.6
0.6 o
-~,
g 04
"i
~
t c~ o.2
0.4-
02
o
o MeasuredValue - - TheoreticalValuq o
a 1.0
2.0
IR-ffee
Voltage
(a)
{v)
~
. . . . 10
'
' 20
lR-free Voltage (v) (b)
Fig. 6. Relation between/R-free voltage and conversion ratio of water to hydrogen obtained experimentally at (a) 859°C and (b) 509°C.
511
DECOMPOSITION OF TRITIATED WATER
0.6
Temperoture (°C) 800
600
1000
t.6 0.4 o
1.4
0.2 ~ { ~ . . . . - - - ' -. . . 8.
011
O. 2
I
1.1
,
, I
1.0 0.9 Reciprocol Temperolure (103/K)
1.2
[
08
'.0
Fig. 9. Temperature dependence of the separation factor for protium(H) and deuterium(D) for the electrolysis reaction shown as an Arrhenius plot. The solid line shows the theoretical value. Figure 8 shows the difference in electrolytic voltages due to isotope effects (ED - EH), which are estimated from the E1/2 values obtained from the electrolysis of D 2 0 and H 2 0 under the same conditions. A theoretical value calculated from (7) is also shown in the figure. In the temperature range 700-900°C, heavy water is decomposed at a higher voltage than light water, and the values E D - - E H were widely scattered around 20 mV. This value is very small compared to the electrolytic voltage of about 1 V. This result suggests that the isotopic difference is negligible in practical applications of the cell. From the figure, the trend of ED - EH decreasing with temperature can be seen. Figure 9 shows the separation factor, o:, for protium(H) and deuterium(D), defined by (9), obtained in the electrolysis of a mixture of heavy and light water in the temperature range 600-900°C. Deuterium was enriched in the vapor phase which remained in the product stream. In the obtained data, oLdid not vary with x. This result can be understood from (8), which indicates that the ratio of PHz/PH20 to PD2/PD2o is constant. Furthermore the separation factor did not vary with the flow rate of reactants. In the figure, the relationship between separation factor and temperature is shown in an Arrhenius plot and is compared to the calculated values. Experimental values were slightly higher than the theoretical ones.
equation. However, a deviation which was thought to be an effect of overpotential was observed in the lower temperature region. These results will be the basis for the design of actual cells and their operational conditions for fusion reactor fuel systems. The isotope effect in this process was measured with heavy water and was found to be minor compared to the total input voltage, but proved to be a little higher than the theoretical value. The difference between isotopes is expected to be negligible, in practice, even with tritium. Reduction of the operating temperature is desirable for reliable operation of the cell with tritium. For this purpose, more conductive materials and improvements in the fabrication technique of the cell are required. Experiments with tritium are also necessary to confirm the feasibility of the cell for the purpose described in this paper. The test is being performed at the Tritium Systems Test Assembly (TSTA), which will demonstrate the fuel cycle for fusion reactors in the Los Alamos National Laboratory. Acknowledgements--The authors are deeply indebted to T. Ozawa, K. Nozaki and A. Negishi (Electrotechnical Laboratory) for their valuable advice. They would also like to thank Y. Obata, K. Iwamoto (Japan Atomic Energy Research Institute), J. L. Anderson, J. R. Bartlit and D. O. Coffin (Los Alamos National Laboratory) for their helpful discussions.
CONCLUSION A solid oxide electrolysis cell was proposed for the decomposition of tritiated water. The feasibility of this method has been verified experimentally by the electrolysis of water vapor in the temperature range 500950°C with high conversion ratios from water to hydrogen up to 99.9% being obtained. The characteristics of the cell have been proved to be described well by the equation deduced from the Nernst
REFERENCES 1. E. C. Kerr, J, R. Bartlit and R. H. Sherman, Fuel cleanup system for the Tritium Systems Test Assembly: design and experiments, Proc. ANS Topl. Meet. Tritium TechnoI. Fission, Fusion and Isotopic Applic., Dayton, Ohio (1981). 2. T. K. Mills, R. E. Ellis and M. L. Rogers, Recovery of tritium from aqueous waste using combined electrolysis catalytic exchange, Proc. ANS Topl. Meet. Tritium Technol, Fission, Fusion and Isotopic Applic., Dayton, Ohio (1981).
512
S. KONISHI, H. OHNO, H. YOSHIDA, H. KATSUTA AND Y. NARUSE
3. S. Konishi, H. Ohno, H. Yoshida and Y. Naruse, Decomposition of tritiated water with solid oxide electrolysis cell, NucL Technol. Fusion 3, 195-198 (1983). 4. H. S. Spacil and C. S. Tedmon Jr., Electrochemical dissociation of water vapor in solid oxide electrolyte cell, J. electrochem. Soc. 116, 1618-1633 (1969).
5. W. Doenitz, R. Schmidberger, E. Steinheil and R. Streicher, Hydrogen production by high temperature electrolysis of water vapor, Int. J. Hydrogen Energy 5, 55-63 (1980).
Int. J. Hydrogen Energy, Vol. 11, No. 8, pp. 507-512, 1986. Printed in Great Britain.
SOLID OXIDE ELECTROLYSIS CELL FOR DECOMPOSITION OF TRITIATED WATER S. KONISHI, H. OHNO*, H. YOSHIDA, H. KATSUTA* and Y. NARUSE Department of Thermonuclear Fusion Research, *Department of Fuels and Materials Research, Japan Atomic Energy Research Institute, Tokai, Ibaraki, 319-11 Japan (Received for publication 10 January 1986) Abstract--The decomposition of tritiated water vapor by means of solid oxide electrolysis cells has been proposed for the application to the D-T fusion reactor system. This method is essentially free from problems such as large tritium inventory, radiation damage, and generation of solid waste, so it is expected to be a promising one. Electrolysis of water vapor in an argon carrier was performed using a tube-type stabilized zirconia cell with porous platinum electrodes over the temperature range 500-950°C. High conversion ratios from water to hydrogen, of up to 99.9%, were achieved. The characteristics of the cell were deduced from the Nernst equation and the conversion ratios expressed as a function of the/R-free voltage. Experimental results agreed with the equation. The isotope effect in electrolysis is also discussed and experiments with heavy water were carried out. The obtained separation factor was slightly higher than the theoretical value.
NOMENCLATURE a H 2, a H 2 0 , a o 2
E Eo
F
£,f AG, A G ° l,P
PH2, PH20, Po2
R R T X O,/
Activities of H2, H 2 0 and 02 required cell voltage for the electrolysis of water required voltage for the electrolysis of water at standard state and temperature T Faraday constant molar flow rates of water vapor at the inlet and outlet of the cell, respectively Gibbs free energy of formation for water or heavy water (standard state) current through the cell in operation and corresponding to the molar flow rate of water into the cell partial pressures of H2, H20 and O2 gas constant [J tool -1K -1] resistance of the cell temperature [K] conversion ratio from water to hydrogen separation factor between protium(H) and deuterium(UD). INTRODUCTION
In deuterium-tritium ( D - T ) fusion reactor systems, tritiated water forms in various processes such as breeding blankets, fuel clean-up systems for the reprocessing of plasma exhaust and recovery processes for tritium leakage from the primary enclosure. It is desirable to decompose this tritiated water and to recover tritium as hydrogen gas for fueling the reactor core. Although some techniques such as activated metal getters [1], conventional wet electrolysis cells and solid polymer electrolyte(SPE) cells can be applied, they suffer the 507
disadvantages of periodic replacement and generation of solid waste, large tritium inventory and radiation damage by tritium [2], respectively. Decomposition of water vapor using solid oxide electrolysis cells has been proposed for this purpose [3]. Such a cell would have a small tritium inventory, high tritium compatibility and would operate continuously in the gas system. This process should produce little radioactive waste because the ceramic electrolyte, through which only oxygen ions can pass and hydrogen permeability is expected to be very small, will not allow contamination of the exhaust oxygen by any significant amounts of tritium if the cell is completely gas tight. This concept has been proposed as a technique for hydrogen production [4, 5] and is intended to be operated at high temperatures around 1000°C. For application to the fusion reactor fuel systems, safe and reliable operation of the cell is important, while efficiency with regard to electrical energy is not essential. In order to design the cell and to determine optimum operating conditions, detailed characteristics of the cell with hydrogen isotopes at temperatures lower than the case for hydrogen production were required. In this paper, characteristics of the solid oxide electrolysis cell are discussed with regard to electrochemical considerations and experimental data. The isotope effect in the electrolysis reaction is also estimated and measured experimentally with protium (light water) and deuterium (heavy water). E Q U A T I O N S F O R THE CELL CHARACTERISTICS An electrolysis cell is shown schematically in Fig. 1. The cell is composed of two electrodes with an oxygen ionic conductor between them. At the cathode, reduction of water vapor: H 2 0 ~ H 2 + 0 2 - proceeds,
508
S. KONISHL H. OHNO, H. YOSHIDA, H. KATSUTA AND Y. NARUSE In terms of currents I = 2F(f0 - f ) , I,i = 2F)~, (4) can also be written
Inlet
gO2= l I
Outlet
E - I R = Eo + (RT/2F) l n ( I / I o - I),
H20 : fo__.. H2 :fo-f
PH20 : Po ...... "v" H20 H2
and the conversion ratio of water to hydrogen, x = ()Co - f ) / f o ,
PH2 : P o l o f
E - I R = E ° + (RT/2F) ln(x/1 - x).
PH20: PO+
The value E - I R is the/R-free voltage of the cell which is related to the composition of the product. It should be noted that the conversion ratio x depends on the/R-free voltage, E - I R , and is not affected by the pressure or flow rate of the reactant. The relationships between conversion ratio x and /R-free voltage, are calculated from (6) and summarized in Fig. 2. As seen in the figure, the conversion ratio rises to 99.9% at about 1.3 V and approaches unity at higher voltage. The values of E ° at each temperature corresponds to the points x = 0.5. Because of the endothermic nature of the reaction,/R-free voltage decreases with increasing temperature. Due to the temperature dependence of the logarithmic term in (6), however, this effect decreases with the value of x. For the various isotopes of hydrogen, only the constant term E ° in (6) is different because of the difference of Gibbs free energy of formation AG °. In the case of electrolysis of HzO and D20, the difference in voltage for the same conversion ratio x is calculated as follows.
Fig. 1. A model of the solid oxide electrolysis cell.
while the anode reaction is formation of oxygen gas: 02-~½02. These reactions are summed to H20 ---*Ha + ½02 and the Gibbs free energy change for the reaction, AG, is, a0.5 aHz oz
AG = AG ° + R T l n ~
(1)
aH20
where AG ° is the standard Gibbs free energy change (per mole) for the reaction HzO--*Hz + ½Oz at temperature T and an2, a02 and all20 are the activities of H2, 02 and H 2 0 in the cell, respectively. This relation can also be written E = E ° + (RT/2F) ln(aH2 a o0.5 JaH2o).
(2)
This equation is equivalent to the Nernst equation for the e.m.f. (electromotive force) of a fuel cell using H 2 and 02, where E ° is the standard e.m.f, of the reaction H2 + ½02---~ H20. Activities of reactants and products are expressed as partial pressures in the cell because the reaction proceeds in the gas phase regardless of the electrolyte. For a cell being operated with current I and apparent resistance R, the ohmic(IR) loss is related to the difference in electrochemical potential of oxygen between anode and cathode. Then (2) is rewritten E = E ° + (RT/2F)ln(Pn2 P ~ / P r h o )
(5)
+ IR
(3)
where Po2 is the partial pressure of 02 at the anode. In the model cell shown in Fig. 1, water vapor of molar flow rate f0 and pressure P0 is reduced to hydrogen gas of flow rate)C0 - f and partial pressure Po(fo - f ) / f o in the cathode compartment. The partial pressure of oxygen at the anode, Po2, is assumed to be unity. The number of molecules does not change in the reaction at the cathode. Gaseous reactants and products are assumed to be mixed well so that almost all the molecules are carried to the reaction point on the cathode surface. In this condition, the gases in the cell are related to the applied voltage on the cathode, and the composition of the gas at the outlet of the cell in the steady state is expressed by (3). Using the molar flow rates, (3) can be written E - I R = E ° + (RT/2F)In(f0 - f / f ) .
(4)
ED -- EH = ( AGo D20 -- AG° n2o)/2F
= 38.3 - 2.49 × 10 -2 T
(6)
(7)
(mV)
where AG~2 o and AG~2 o are the Gibbs free energy of formation for D20 and H20, respectively. The isotope effect in the electrolysis of water is known to be very large compared with other processes. However, as seen from (7), the effect decreases with the temperature and is not so significant at high temperatures. It should be noted that the difference AG~2 o - A G ~ 2 o also expresses the equilibrium constant for the exchange reaction D2 + H 2 0 ~ H 2 + D 2 0 as, AG~2o - AG~I2O = -RTln(Pr~z. P H 2 o ) / ( P D 2 o .
PHz).
(8)
This relationship indicates that the isotope effect in electrolysis is equivalent to that in the isotope exchange reaction. The separation factor o~ is defined as the ratio of isotopes in the vapor phase (H20, HDO, D20) to those in the hydrogen gas phase (H2, HD, D2) in the product stream by o~= (Hgas/Dgas)/(H,,p/Ovap) ( P n z + ½PHD) (PD20 + ½PHDo)
-- ( e o z + ½PHD) (PHzo + ½PHDo)
(9)
This factor can be related to the (7) and is expected to decrease with temperature.
DECOMPOSITION OF TRITIATED WATER
"T -
,oo/////// o.,
measured by this device. The cell was operated in the temperature range 500-950°C. Temperature was measured with a Pt vs 13%Rh-Pt thermocouple, The isotope effect in the electrolysis reaction was measured with D20 or mixtures of D20 and H20. The quantitative analysis of protium(H) and deuterium(D) in the product stream was made using a Mass Spectrometer and Multi-Ion Selector (ANELVA TE-150 and MIS-200).
Temperoture 900800_~/////~,,,///// zoo-_/j_y//ll
o o
o
. . . . . .
/ ////I/1.; / / / / t /" 50o
;
509
RESULTS AND DISCUSSIONS 1.2
IR-free
Vollege
1.3
(V)
Fig. 2. Relationships between/R-free voltage and conversion ratio of water to hydrogen obtained numerically.
EXPERIMENTAL Zirconia cells stabilized with calcia, yttria and other rare earth oxides were prepared as electrolytes. In this study, most of the experiments were carried out with zirconia stabilized with 8 mol% yttria, this being the most proven and conductive material. The electrolytes were frabricated into the shape of a 13 mm o.d., 9 mm i.d. test tube. On the inner and the outer surface, porous electrodes of about 20 cm 2 were formed by baking platinum paste which had been doped onto the surfaces. The arrangement of the experimental apparatus is shown in Fig. 3. The electrolysis cell was mounted in a quartz tube and heated by an electric furnace. Reactants and products were carried by an argon stream with a flow rate regulated to 10-300 cm 3 min-Z. Concentration of water vapor, from which the conversion ratio was estimated, was measured with hygrometers (Panametrics-700) at the inlet and the outlet of the cell. A potentiostat (Hokuto Denko HA-301), which supplies stable cell voltages with errors less than 0.1%, was used as a power source. The electrolysis current was also O~t
Variation of the cell resistance with temperature is shown in Fig. 4. Conductivity can be written in an Arrhenius form, though in the high temperature region, a residual resistance of about 2 ~ was observed. This seemed to be caused by contact resistance at the interface between electrodes and electrolyte. Some examples of relationships between voltages and current during electrolysis are shown in Figs 5a, b obtained at 859°C and 506°C, respectively. Closed circles represent values for the cell voltage and open circles the/R-free voltage, ( E - I R ) , estimated from cell voltage, resistance and current. Figures 6a, b show the relationships between cell voltage and the conversion ratio from water to hydrogen, compared with the prediction from (6) as indicated by the solid lines. The highest values of the conversion ratio x were about 95% for lower temperatures and up to 99.95% for high temperatures. The deviation of conversion ratio from unity is caused by residual water in the tubing between the cell and the outlet hygrometer. This was calculated as water which failed to be decomposed by electrolysis. As seen in Fig. 6a, the experimental results agreed well with the theoretical values calculated from (6) at high temperatures. The discrepancy observed in Fig. 6b indicates the existence of an overpotential neglected in (6).
Ternperoture 900 800 700 600
(°C)
500
400
103
H2
H~ (
Slobilized ; Pf - elecl 310
Ele Fur
O
Gos ,
F~, F2 : Moss Flow Meter H~, He : Hygrorneler Fig. 3. Experimental set-up of electrolysis cell.
0.8
I
1.0
,
I
1.2
,
1
1.4
,
]
1.6
Reoiprocol Temperofure IOOO/T (K-I) Fig. 4. Apparent resistance of the electrolysis cell shown against the reciprocal temperature.
510
S. KONISHI, H. OHNO, H. YOSHIDA, H. KATSUTA AND Y. NARUSE 250
'
ioo o
859 °C
E200
• 6
'
0
•
o
O
1.2
o
O
o
~ is0 ~g
LLI
o o
. ~ 100
o
•
o
• Cell Voltage
•
o oe
•
f.0
0.5
1.0
IR-free Voltage
o
'7
I 20
1/5
25
Voltage (v)
2O
r
( o o o o
E f5
0
0
0
I
f000
Fig. 7. Temperature dependence of the value of EV2, i.e. the/R-free voltages corresponding to x = 0.5, which are the experimental values of E ~ = AG°/2F. The solid line shows the theoretical value.
•
• •
OT O •
015
J
Temperature (°C)
•
o
0
I
800
•
O
o
J
• •
g
o
I
600
•1
oo
509°C
l
080o
(0}
'0
• Cell Voltage o IR-free Voltage 'l~
~ l0
6O
~~
Voltage (v) 40
(b) Fig. 5. Voltage-current characteristics of the cell during electrolysis at (a) 859°C and (b) 509°C. The open circles shows the /R-free voltages and the closed circles show the cell voltage. This fact suggests that the reaction on the cathode surface is very fast and is reversible, perhaps due to the catalytic activity of the platinum electrodes and/or the very high temperatures. Figure 7 shows the values of E1/2, which are obtained from t h e / R - f r e e voltages for x = 1/2, compared to the theoretical value, E °. Though the data are scattered, the deviation decreases with
temperature.
I
2O
0 0
0
-----"0 . . . . . .
0
o
--
.~
~_~.~
O
0 -20
I
I
I
700
800
900
Temperature
(°C)
Fig. 8. Differences in the /R-free voltage for electrolysis of heavy water and light water. The solid line shows the theoretical value. fO I
1.0
I
~
I 0 ~
o
l
I
509°C
o 08
0.8
-io
5-
° o
o
0.6
0.6 o
-~,
g 04
"i
~
t c~ o.2
0.4-
02
o
o MeasuredValue - - TheoreticalValuq o
a 1.0
2.0
IR-ffee
Voltage
(a)
{v)
~
. . . . 10
'
' 20
lR-free Voltage (v) (b)
Fig. 6. Relation between/R-free voltage and conversion ratio of water to hydrogen obtained experimentally at (a) 859°C and (b) 509°C.
511
DECOMPOSITION OF TRITIATED WATER
0.6
Temperoture (°C) 800
600
1000
t.6 0.4 o
1.4
0.2 ~ { ~ . . . . - - - ' -. . . 8.
011
O. 2
I
1.1
,
, I
1.0 0.9 Reciprocol Temperolure (103/K)
1.2
[
08
'.0
Fig. 9. Temperature dependence of the separation factor for protium(H) and deuterium(D) for the electrolysis reaction shown as an Arrhenius plot. The solid line shows the theoretical value. Figure 8 shows the difference in electrolytic voltages due to isotope effects (ED - EH), which are estimated from the E1/2 values obtained from the electrolysis of D 2 0 and H 2 0 under the same conditions. A theoretical value calculated from (7) is also shown in the figure. In the temperature range 700-900°C, heavy water is decomposed at a higher voltage than light water, and the values E D - - E H were widely scattered around 20 mV. This value is very small compared to the electrolytic voltage of about 1 V. This result suggests that the isotopic difference is negligible in practical applications of the cell. From the figure, the trend of ED - EH decreasing with temperature can be seen. Figure 9 shows the separation factor, o:, for protium(H) and deuterium(D), defined by (9), obtained in the electrolysis of a mixture of heavy and light water in the temperature range 600-900°C. Deuterium was enriched in the vapor phase which remained in the product stream. In the obtained data, oLdid not vary with x. This result can be understood from (8), which indicates that the ratio of PHz/PH20 to PD2/PD2o is constant. Furthermore the separation factor did not vary with the flow rate of reactants. In the figure, the relationship between separation factor and temperature is shown in an Arrhenius plot and is compared to the calculated values. Experimental values were slightly higher than the theoretical ones.
equation. However, a deviation which was thought to be an effect of overpotential was observed in the lower temperature region. These results will be the basis for the design of actual cells and their operational conditions for fusion reactor fuel systems. The isotope effect in this process was measured with heavy water and was found to be minor compared to the total input voltage, but proved to be a little higher than the theoretical value. The difference between isotopes is expected to be negligible, in practice, even with tritium. Reduction of the operating temperature is desirable for reliable operation of the cell with tritium. For this purpose, more conductive materials and improvements in the fabrication technique of the cell are required. Experiments with tritium are also necessary to confirm the feasibility of the cell for the purpose described in this paper. The test is being performed at the Tritium Systems Test Assembly (TSTA), which will demonstrate the fuel cycle for fusion reactors in the Los Alamos National Laboratory. Acknowledgements--The authors are deeply indebted to T. Ozawa, K. Nozaki and A. Negishi (Electrotechnical Laboratory) for their valuable advice. They would also like to thank Y. Obata, K. Iwamoto (Japan Atomic Energy Research Institute), J. L. Anderson, J. R. Bartlit and D. O. Coffin (Los Alamos National Laboratory) for their helpful discussions.
CONCLUSION A solid oxide electrolysis cell was proposed for the decomposition of tritiated water. The feasibility of this method has been verified experimentally by the electrolysis of water vapor in the temperature range 500950°C with high conversion ratios from water to hydrogen up to 99.9% being obtained. The characteristics of the cell have been proved to be described well by the equation deduced from the Nernst
REFERENCES 1. E. C. Kerr, J, R. Bartlit and R. H. Sherman, Fuel cleanup system for the Tritium Systems Test Assembly: design and experiments, Proc. ANS Topl. Meet. Tritium TechnoI. Fission, Fusion and Isotopic Applic., Dayton, Ohio (1981). 2. T. K. Mills, R. E. Ellis and M. L. Rogers, Recovery of tritium from aqueous waste using combined electrolysis catalytic exchange, Proc. ANS Topl. Meet. Tritium Technol, Fission, Fusion and Isotopic Applic., Dayton, Ohio (1981).
512
S. KONISHI, H. OHNO, H. YOSHIDA, H. KATSUTA AND Y. NARUSE
3. S. Konishi, H. Ohno, H. Yoshida and Y. Naruse, Decomposition of tritiated water with solid oxide electrolysis cell, NucL Technol. Fusion 3, 195-198 (1983). 4. H. S. Spacil and C. S. Tedmon Jr., Electrochemical dissociation of water vapor in solid oxide electrolyte cell, J. electrochem. Soc. 116, 1618-1633 (1969).
5. W. Doenitz, R. Schmidberger, E. Steinheil and R. Streicher, Hydrogen production by high temperature electrolysis of water vapor, Int. J. Hydrogen Energy 5, 55-63 (1980).