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Proton conducting (Na, H)5GdSi4O12 ceramics and their use in steam electrolytes
Solid State Ionies 20 (1986) 147-151 North-Holland, Amsterdam
PROTON CONDUCtiNG (Na, H)5 GdSi4012 CERAMICS AND THEIR USE IN STEAM ELECTROLYTES Ki/ro
YAMASHITA * and Patrick S. NICHOLSON
Ceramic Engineering Research Group, Department of Metallurgy and Materials Science, McMaster University, Hamilton, Ontario, Canada Received 16 August 1985 Accepted for publication 24 October 1985
A polyerystalline proton conductor has been obtained by field-assisted ion exchange (FAIE) of the mobile Na+ in Na5 GdSi4012 by Ha O+. Polyerystalline Na5 GdSi40 j2 (NGS) and (KxNa $-x)GdSi4Ol2 (KNGS) were examined as preettrsor ceramics. By modelling the FAIE process, it appears that KNGS is superior to NGS for ion exchange. The time required to complete the ion exchange process is ~1.7 × 102 hat 338 K, and the activation energy for proton conduction
is 10.7 ± 0.3 (kcal/mol). The HsO-NGS ceramicwas successfullyincorporated into steam electrolysiscell operating at -90°C.
1. Introduction Although hydrous compounds of some salts [1,2] ~ d single crystals such as/3-alumina [3-5 ] exhibit high protonic conduction, there are few reports of polycrystalline proton conductors. The water molecules contributing to protonic conduction are lost during sintering at temperatures >100°C. An alternative synthesis method [6,7] was developed wherein the mobile ions in the sinter are exchanged with proton hydrates (H+ (H20)n) via field-assisted ion-exchange (FAIE). H20 loss is not involved in this process. In the present study, Na 5 (GdY)Si4OI1 (NGS, NYS) ceramic precursors were investigated and their protonic conduction characteristics explored and verified by steam electrolysis.
lined previously [8-10]. Isostatically pressed pellets of calcined powders were sintered at 1050°C (NGS) and 1140°C (NYS) to give >96% theoretical density. NKGS was obtained by the conversion of NGS in NaC1/KC1 meRs at 800°C [7]. To avoid cracking, the ion-exchange was done by immersion in consecutive melts of low to high [K+ ] (= ca. 0.5). HNGS was prepared from both NGS and NKGS in 0.1 M CH3COOH by FAIE [6,7] at 25-90°C. To study Na+ and K + ionic motion differences in the NGS-type structure, ac measurements were cartied out using the 2-probe method with non-blocking electrodes. The feasibility of using HNGS as an electrolyte for steam electrolysis was demonstrated at 90°C utilising the cell design shown in fig. 1. Platinum electrodes were used in this cell.
3. Results and discussion 2. Experimental procedures Dense NGS and NYS sinters were produced as out* Presently at Department of Industrial Chemistry, Faculty of Technology, Tokyo Metropolitan University, Fukasawa, Setagaya-ku, Tokyo 158, Japan.
0 167-2738/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
42 of the 90 Na+ ions (47%) in the unit cell of NGS are considered mobile [11]. Taking this into account, ion-exchange was conducted in NaC1/KC1 melts with maximum [Na+]/[Na + ] + [K+ ] = 0.5. This ensured the introduction o f K + ions onto mobile Na +. sites exclusively (this procedure assumes Raoultian
148
K. Yamashita, P.S. Nicholson /Proton conducting (Na, H) s GdSi 4012 ceramics
1 H2 Gas 2 H2 Collector
3 H20 4 Shielding Cement S Pt Electrode e Electrolyte ? DC Power Supply 8 Cooling Water 9 Heater
ITI Steam
Boiling
-a_0_0&0_0_L:® Fig. 1. Schematic representation oi" steam electrolysis unit.
behaviour of the NGS. The actual solution behaviour of this material is unknown). The replacement of K+ with Na+ was subsequently conf'nvned by EPMA [7]. Because K+ is larger than Na+, an electrical conductivity difference between KNGS and NGS is expected (fig. 2). KNGS has an activation energy for the electrical conduction of 6.3 kcal/mol. This value is higher than the 5.9 and 4.5 kcal/mol values for NGS and NYS, respectively. The process of ion exchange (K+/Na+-H30 +) can be understood via the following model. If one assumes that (1) the guest cations (G+), (hereafter G+ = H+ or H30+), migrate through the sites left by the host cations (M+); (2) G+ does not precede M+ and (3) the boundary between the ion-exchanged (or protonated) layer and the unexchanged layer is steep. Assumptions (1) and (2) are equivalent to asserting
that the mobility of G+, (pg), is smaller than that of M+, (/~m) and the summation of(C G - CM) (the concentrations of G+ and M+) = CO and the total concentration of sites (per unit area) allowed for mobile ions is constant. From assumption (3), the velocity of the boundary, v, will be characteristic of the FAIE process. Although concentration gradient-driven diffusion should be taken into account for precise analysis, low temperatures will limit diffusion without field assistance and this contribution will be assumed negligible. For simplicity, PG and PM are assumed independent of concentration. The transported charge through unit area per second =I/F=Cov
or I = C o v F
EblG laM C 0 LlaM + (L O - L ) p G '
(1)
149
K. Yamashita, P.S. Nicholson /Proton conducting (Na, H) s GdSi4 012 ceramics
r (oc) 340 320 w
300 280
250
15
Conductivity of host ions
~
>x lO
2.0
,< 1.8
Ko( Conductivityo~ guest
~K 5
.. I
1.6
~.10 ~K-10 .A °o ~
K=5
0
0 i
c
ions
KNGS
1.4
I 0.2
I 0.4
I 0.6
I G8
1.0
X=1/1 o
Fig. 3. Ion exchangefront velocityversus mobility ratio of host to guest cations.
0 0
1.2
(/zM -- pG)L 2 + 21h3Lo L - 2E/aG/aMt = 0 ,
(3)
1.0
1.,5
I 1.6
I
I
!
1.7
1.8
1.9
,0%
and the time required for completion of FAIE, tF, resuits when L = L O in eq. (3), i.e.
(K-')
L2 L2 tF = (~---~) (1 + ~M ) = ( ~ - ~ )
Fig. 2. ac conductivityof NYS, NGS and KNGSversus lIT. where I is the current density, F the Faraday constant, E the constant potential across a specimen, L the thickness of the protonated layer and L O the sample thickness. From eq. (1), it follows that v = laGE/L 0 { ( L / L o ) + [(LO - L)/L 0 ] ( J / G / / a M ) )
-1
,
(2)
where ( L / L o ) and (LO - L)/L 0 are the ratios ofprotonated layer and unexchanged layer to the total thickness. The rate of ion-exchange can be estimated from (2). The dependence o f v on/aG//aM is shown in fig. 3. In this figure o = [x + (l/k)(1 - x ) ] - I is plotted versus x (= L/L o : the protonation ratio) and k (=/aM//aG : the ratio of the mobilities), oO = (/agE/Lo) is the assumed velocity of G+ neglecting /aM. o decreases as k approaches unity, i.e., the conductivity of the two ions approaches equality. The k dependence of v is marked in the initial stages of FAIE (x < 0.2). Integrating (2) with respect to t gives
(1 + k ) .
(4)
l f k >> 10, t F is independent o f k a n d = (L20/[2E/aM]). From eq. (1) the current/time relationship during FAIE is I = (CoFE)/{(LO//aM)2 + 2 [(1//aG) -- (1//aM)]Et}ll2
(5) or, neglecting the term (Lo//aM)2 I = (CoFE)/{2[(1/Ih3) - (1//aM)]Et)l/2
(6)
or in terms of resistivity Co), realizing that p =
1/CoF.,
I = [(CoFE)/2(p G - PM)] t12(1/tl/2) •
(7)
Eq. (5) is equivalent to the expression given by Baucke [12,13]. By plotting/versus 1/t 1/2, PG -- PM can be determined from the slope K = [ECoF/2(p G - pM)] 1/2 ,
i.e., PG - - PM = (ECo F)/(2K2) or PG = (ECoF)](2K2) when PG ~" PM [6]. The FAIE of NGS as host structure, gives a linear I venus l [ t l l 2 relationship intercepting the origin [6,7], since PG for Na+ and
150
K. Yamashita, P.S. Nicholson/Proton conducting (Na, HJs C'dSi4012 ceramics
• 25oc
lO
o:7
/ t l
12
8
8
4 4
2
OJ
1D
1.5
2.0
I 1.0
2.5
TIME,t
T I M E , t (se¢ x 1.04 )
(H+ + (H~O)n) are 102-103 ~ cm [14,15] and 106 "" 105 ~2 cm [7], respectively (25 to 90°C). When/aG is close to/aM (i.e.,/aG/PM ca. 10-3), the terms (Lo//aM) 2 and toM cannot be neglected. For FAIE of KNGS, eq. (5) may be rewritten as: 2(p G -- PM)E
-ff-\~r2 ] + (Eltr2)2Co F t ,
! 3.0
(sec x 105)
Fig. 5. Temperature dependenceof current changeversus time (1//2 versust) (from eq. (8)).
Fig. 4.1//2 versus t plot for KNGS (voltage = 10 V de).
1 _(/aMLo]2
| 2.0
(5')
where r is the radius of a specimen. The plot of 1/12 versus t (fig. 4) gives/a M and/aG from the slope and the intercept. The calculated results are listed in table 1 (CoFwas taken as 1250 coul./cm 3 [7]). Another method of estimating the temperature dependence of/aG is to FAIE for a given time at temperature Ti, and then continue at a different temperature Ti+ 1" The following equation is obtained for the (i + l)th FAIE
1_
,+½((P )i+li2
12
(.r2~2CoF
I~('--~G)i]
where Ii is the finalcurrent at the temperature Ti, and (Po)i and (PG)i+ 1 the resistivitiesof the protonated layer at temperatures T i and Ti+ I, respectively.The time t for the (i + l)th FAIE must be computed from 0. By plotting I/I2 versus t, (PG)i+I can be obtained via the intercept,//and (PG)i" (Fig. 5 for KNGS at 25°C and 55°C). (PG)298 is calculated from eq. (5') and (/aG)328 from eq. (8) and (/aG)298. The calculated values are listed in table 1. The conductivity results are shown in fig. 6. The resulting activation energies for the individual cations are listed in table 2. The guest cations are considered to be hydrated protons (H30+). The activation energy for conduction of H 30 + through KNGS is slightly smaller than through NGS. Clearly the pre-expansion
Table 1 Calculated resistivities (the equations used, refer to numbers in the paper). M or G
Tempexatttre (K)
P M Na+, K+
298 338 298 338 328
PG H + (H20) n
(8)
Equations used
(5') (8)
Resistivity (~ ern) 2.2 X 5.7 X 6.9 x 1.0 X 1.5 X
I0 s 10 4 106 10 6 10 6
K. Yamashita, P.S. Nicholson /Proton conducting (Na, H) s GdSi4012 ceramics
Table 3 Time to complete the FAIE process (the thickness of a specimen = 0.1 cm; applied voltage = 10 V; charge density = 1254 (ceul./ern)
T (°C) 75
eo
26
Material
Temperature (K)
tF (h)
NGS a)
298 328 356
3.1 X 103 7.3 X 102 1.7 X 102
KNGS
298 328 338
1.2 X 10a 2.6 X 102 1.7 X 102
NGS : Na+
7
-2
5 (~
-4
~
151
))n
a) Taken fxom ref. [7]. NGS: H~H20)n
-6
t 2.8
I 3.0
! 3.2 zO a
T
I 3.4
(K " l )
Fig. 6. The calculated conductivities for NGS-type structures. of the network lattice b y the larger K + ions is an advantage. In fact, NGS specimens often cracked during the final stages of the FAIE process. The conductivity of K + through the NGS skeleton has an activation energy of 8.35 (kcal/mol) but lower than that of H3 O+ for Na + (6.86 kcal/mol) b u t lower than that of H3 O+ (10.7 + 0.3 (kcal/mol) between room temperature and 350°K. The time for completion of the FAIE process (table 3) was estimated using eq. (4) and the results in table 2. Higher temperatures accelerate the process. At 3 3 0 - 3 4 0 K, t F is 1 - 2 X 102 h for polycrystalllne NGS. A steam electrolysis cell was constructed using the resulting HNGS electrolyte. The electrolyte area was 0.935 cm 2 and its thickness 1.54 mm. The cell potenTable 2 Calculated activation energies for the conduction of various cations through NGS-type structures. Mobile ion
Skeleton
Activation energy (kcal/mol)
Ref.
Na+ Na+ Na+, K+ H+(H20)n H+(H20)n
NYS NGS KNGS NGS KNGS
8.58 6.86 8.35 10.98 10.68
[ 15 ] [ 14 ] this work this work this work
tial was maintained constant at 10 V and a current of 10 + 3 mA passed for 100 h. The hydrogen produced was collected over water. The cell eventually ceased to operate due to RTV-cement seal failure. The operating temperature was ~90°C.
References [ 1] W.A. England, M.G. Cross, A. Hamnett, P.H. Wiseman and J.B. Goodenough, Solid State Ionics 1 (1980) 231. [2] U. Chowdhury, J.R. Barkley, A.D. English and A.W. Sleight, Mater. Res. Bull. 17 (1982) 917. [3] G.C. Faxdngton, J.L. Briant and H.S. Story, Electrochim. Acta 24 (1979) 769. [4] G.C. Farrington and J.L. Briant, Mater. Res. Bull. 13 (1978) 763. [5 ] N. Baffler, J.C. Badot and Ph. Colomhan, Solid State Ionics 2 (1980) 107. [6] M. Nagai and P.S. Nicholson, Solid State Ionics 15 (1985) 311. [7] K. Yamashita and P.S. Nichoison, Solid State Ionics 17 (1985) 121. [8] J.J. Bentzen and P.S. Nicholson, Mater. Res. Bull. 15 (1980) 1737. [9] J.J. Bentzen and P.S. Nicholson, Mater. Res. Bull. 17 (1982) 541. [10] K. Yamashita and P.S. Nicholson, Solid State Ionics ' 17 (1985) 115. [11] H.U. Beyeler, R.D. Shannon and H.Y. Chen, Solid State Ionics 3/4 (1981) 223. [12] F.G.K. Baucke, in: Materials science reseaxch, eds. A.R. Cooper and AH. Heuer (Plenum, New York, 1975) p. 337. [13] F.G.K. Baucke, J. Non-Ctyst. Solids 40 (1980) 159. [14] R.D. Shannon, B.E. Taylor, T.E. Gier, H-Y. Chen and T. Berzins, Inorg. Chem. 17 (1978) 958. [15] H.Y-P. Hong, LA. Kafala and M. Bayard, Mater. Res. BUlL 13 (1978) 757.
PROTON CONDUCtiNG (Na, H)5 GdSi4012 CERAMICS AND THEIR USE IN STEAM ELECTROLYTES Ki/ro
YAMASHITA * and Patrick S. NICHOLSON
Ceramic Engineering Research Group, Department of Metallurgy and Materials Science, McMaster University, Hamilton, Ontario, Canada Received 16 August 1985 Accepted for publication 24 October 1985
A polyerystalline proton conductor has been obtained by field-assisted ion exchange (FAIE) of the mobile Na+ in Na5 GdSi4012 by Ha O+. Polyerystalline Na5 GdSi40 j2 (NGS) and (KxNa $-x)GdSi4Ol2 (KNGS) were examined as preettrsor ceramics. By modelling the FAIE process, it appears that KNGS is superior to NGS for ion exchange. The time required to complete the ion exchange process is ~1.7 × 102 hat 338 K, and the activation energy for proton conduction
is 10.7 ± 0.3 (kcal/mol). The HsO-NGS ceramicwas successfullyincorporated into steam electrolysiscell operating at -90°C.
1. Introduction Although hydrous compounds of some salts [1,2] ~ d single crystals such as/3-alumina [3-5 ] exhibit high protonic conduction, there are few reports of polycrystalline proton conductors. The water molecules contributing to protonic conduction are lost during sintering at temperatures >100°C. An alternative synthesis method [6,7] was developed wherein the mobile ions in the sinter are exchanged with proton hydrates (H+ (H20)n) via field-assisted ion-exchange (FAIE). H20 loss is not involved in this process. In the present study, Na 5 (GdY)Si4OI1 (NGS, NYS) ceramic precursors were investigated and their protonic conduction characteristics explored and verified by steam electrolysis.
lined previously [8-10]. Isostatically pressed pellets of calcined powders were sintered at 1050°C (NGS) and 1140°C (NYS) to give >96% theoretical density. NKGS was obtained by the conversion of NGS in NaC1/KC1 meRs at 800°C [7]. To avoid cracking, the ion-exchange was done by immersion in consecutive melts of low to high [K+ ] (= ca. 0.5). HNGS was prepared from both NGS and NKGS in 0.1 M CH3COOH by FAIE [6,7] at 25-90°C. To study Na+ and K + ionic motion differences in the NGS-type structure, ac measurements were cartied out using the 2-probe method with non-blocking electrodes. The feasibility of using HNGS as an electrolyte for steam electrolysis was demonstrated at 90°C utilising the cell design shown in fig. 1. Platinum electrodes were used in this cell.
3. Results and discussion 2. Experimental procedures Dense NGS and NYS sinters were produced as out* Presently at Department of Industrial Chemistry, Faculty of Technology, Tokyo Metropolitan University, Fukasawa, Setagaya-ku, Tokyo 158, Japan.
0 167-2738/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
42 of the 90 Na+ ions (47%) in the unit cell of NGS are considered mobile [11]. Taking this into account, ion-exchange was conducted in NaC1/KC1 melts with maximum [Na+]/[Na + ] + [K+ ] = 0.5. This ensured the introduction o f K + ions onto mobile Na +. sites exclusively (this procedure assumes Raoultian
148
K. Yamashita, P.S. Nicholson /Proton conducting (Na, H) s GdSi 4012 ceramics
1 H2 Gas 2 H2 Collector
3 H20 4 Shielding Cement S Pt Electrode e Electrolyte ? DC Power Supply 8 Cooling Water 9 Heater
ITI Steam
Boiling
-a_0_0&0_0_L:® Fig. 1. Schematic representation oi" steam electrolysis unit.
behaviour of the NGS. The actual solution behaviour of this material is unknown). The replacement of K+ with Na+ was subsequently conf'nvned by EPMA [7]. Because K+ is larger than Na+, an electrical conductivity difference between KNGS and NGS is expected (fig. 2). KNGS has an activation energy for the electrical conduction of 6.3 kcal/mol. This value is higher than the 5.9 and 4.5 kcal/mol values for NGS and NYS, respectively. The process of ion exchange (K+/Na+-H30 +) can be understood via the following model. If one assumes that (1) the guest cations (G+), (hereafter G+ = H+ or H30+), migrate through the sites left by the host cations (M+); (2) G+ does not precede M+ and (3) the boundary between the ion-exchanged (or protonated) layer and the unexchanged layer is steep. Assumptions (1) and (2) are equivalent to asserting
that the mobility of G+, (pg), is smaller than that of M+, (/~m) and the summation of(C G - CM) (the concentrations of G+ and M+) = CO and the total concentration of sites (per unit area) allowed for mobile ions is constant. From assumption (3), the velocity of the boundary, v, will be characteristic of the FAIE process. Although concentration gradient-driven diffusion should be taken into account for precise analysis, low temperatures will limit diffusion without field assistance and this contribution will be assumed negligible. For simplicity, PG and PM are assumed independent of concentration. The transported charge through unit area per second =I/F=Cov
or I = C o v F
EblG laM C 0 LlaM + (L O - L ) p G '
(1)
149
K. Yamashita, P.S. Nicholson /Proton conducting (Na, H) s GdSi4 012 ceramics
r (oc) 340 320 w
300 280
250
15
Conductivity of host ions
~
>x lO
2.0
,< 1.8
Ko( Conductivityo~ guest
~K 5
.. I
1.6
~.10 ~K-10 .A °o ~
K=5
0
0 i
c
ions
KNGS
1.4
I 0.2
I 0.4
I 0.6
I G8
1.0
X=1/1 o
Fig. 3. Ion exchangefront velocityversus mobility ratio of host to guest cations.
0 0
1.2
(/zM -- pG)L 2 + 21h3Lo L - 2E/aG/aMt = 0 ,
(3)
1.0
1.,5
I 1.6
I
I
!
1.7
1.8
1.9
,0%
and the time required for completion of FAIE, tF, resuits when L = L O in eq. (3), i.e.
(K-')
L2 L2 tF = (~---~) (1 + ~M ) = ( ~ - ~ )
Fig. 2. ac conductivityof NYS, NGS and KNGSversus lIT. where I is the current density, F the Faraday constant, E the constant potential across a specimen, L the thickness of the protonated layer and L O the sample thickness. From eq. (1), it follows that v = laGE/L 0 { ( L / L o ) + [(LO - L)/L 0 ] ( J / G / / a M ) )
-1
,
(2)
where ( L / L o ) and (LO - L)/L 0 are the ratios ofprotonated layer and unexchanged layer to the total thickness. The rate of ion-exchange can be estimated from (2). The dependence o f v on/aG//aM is shown in fig. 3. In this figure o = [x + (l/k)(1 - x ) ] - I is plotted versus x (= L/L o : the protonation ratio) and k (=/aM//aG : the ratio of the mobilities), oO = (/agE/Lo) is the assumed velocity of G+ neglecting /aM. o decreases as k approaches unity, i.e., the conductivity of the two ions approaches equality. The k dependence of v is marked in the initial stages of FAIE (x < 0.2). Integrating (2) with respect to t gives
(1 + k ) .
(4)
l f k >> 10, t F is independent o f k a n d = (L20/[2E/aM]). From eq. (1) the current/time relationship during FAIE is I = (CoFE)/{(LO//aM)2 + 2 [(1//aG) -- (1//aM)]Et}ll2
(5) or, neglecting the term (Lo//aM)2 I = (CoFE)/{2[(1/Ih3) - (1//aM)]Et)l/2
(6)
or in terms of resistivity Co), realizing that p =
1/CoF.,
I = [(CoFE)/2(p G - PM)] t12(1/tl/2) •
(7)
Eq. (5) is equivalent to the expression given by Baucke [12,13]. By plotting/versus 1/t 1/2, PG -- PM can be determined from the slope K = [ECoF/2(p G - pM)] 1/2 ,
i.e., PG - - PM = (ECo F)/(2K2) or PG = (ECoF)](2K2) when PG ~" PM [6]. The FAIE of NGS as host structure, gives a linear I venus l [ t l l 2 relationship intercepting the origin [6,7], since PG for Na+ and
150
K. Yamashita, P.S. Nicholson/Proton conducting (Na, HJs C'dSi4012 ceramics
• 25oc
lO
o:7
/ t l
12
8
8
4 4
2
OJ
1D
1.5
2.0
I 1.0
2.5
TIME,t
T I M E , t (se¢ x 1.04 )
(H+ + (H~O)n) are 102-103 ~ cm [14,15] and 106 "" 105 ~2 cm [7], respectively (25 to 90°C). When/aG is close to/aM (i.e.,/aG/PM ca. 10-3), the terms (Lo//aM) 2 and toM cannot be neglected. For FAIE of KNGS, eq. (5) may be rewritten as: 2(p G -- PM)E
-ff-\~r2 ] + (Eltr2)2Co F t ,
! 3.0
(sec x 105)
Fig. 5. Temperature dependenceof current changeversus time (1//2 versust) (from eq. (8)).
Fig. 4.1//2 versus t plot for KNGS (voltage = 10 V de).
1 _(/aMLo]2
| 2.0
(5')
where r is the radius of a specimen. The plot of 1/12 versus t (fig. 4) gives/a M and/aG from the slope and the intercept. The calculated results are listed in table 1 (CoFwas taken as 1250 coul./cm 3 [7]). Another method of estimating the temperature dependence of/aG is to FAIE for a given time at temperature Ti, and then continue at a different temperature Ti+ 1" The following equation is obtained for the (i + l)th FAIE
1_
,+½((P )i+li2
12
(.r2~2CoF
I~('--~G)i]
where Ii is the finalcurrent at the temperature Ti, and (Po)i and (PG)i+ 1 the resistivitiesof the protonated layer at temperatures T i and Ti+ I, respectively.The time t for the (i + l)th FAIE must be computed from 0. By plotting I/I2 versus t, (PG)i+I can be obtained via the intercept,//and (PG)i" (Fig. 5 for KNGS at 25°C and 55°C). (PG)298 is calculated from eq. (5') and (/aG)328 from eq. (8) and (/aG)298. The calculated values are listed in table 1. The conductivity results are shown in fig. 6. The resulting activation energies for the individual cations are listed in table 2. The guest cations are considered to be hydrated protons (H30+). The activation energy for conduction of H 30 + through KNGS is slightly smaller than through NGS. Clearly the pre-expansion
Table 1 Calculated resistivities (the equations used, refer to numbers in the paper). M or G
Tempexatttre (K)
P M Na+, K+
298 338 298 338 328
PG H + (H20) n
(8)
Equations used
(5') (8)
Resistivity (~ ern) 2.2 X 5.7 X 6.9 x 1.0 X 1.5 X
I0 s 10 4 106 10 6 10 6
K. Yamashita, P.S. Nicholson /Proton conducting (Na, H) s GdSi4012 ceramics
Table 3 Time to complete the FAIE process (the thickness of a specimen = 0.1 cm; applied voltage = 10 V; charge density = 1254 (ceul./ern)
T (°C) 75
eo
26
Material
Temperature (K)
tF (h)
NGS a)
298 328 356
3.1 X 103 7.3 X 102 1.7 X 102
KNGS
298 328 338
1.2 X 10a 2.6 X 102 1.7 X 102
NGS : Na+
7
-2
5 (~
-4
~
151
))n
a) Taken fxom ref. [7]. NGS: H~H20)n
-6
t 2.8
I 3.0
! 3.2 zO a
T
I 3.4
(K " l )
Fig. 6. The calculated conductivities for NGS-type structures. of the network lattice b y the larger K + ions is an advantage. In fact, NGS specimens often cracked during the final stages of the FAIE process. The conductivity of K + through the NGS skeleton has an activation energy of 8.35 (kcal/mol) but lower than that of H3 O+ for Na + (6.86 kcal/mol) b u t lower than that of H3 O+ (10.7 + 0.3 (kcal/mol) between room temperature and 350°K. The time for completion of the FAIE process (table 3) was estimated using eq. (4) and the results in table 2. Higher temperatures accelerate the process. At 3 3 0 - 3 4 0 K, t F is 1 - 2 X 102 h for polycrystalllne NGS. A steam electrolysis cell was constructed using the resulting HNGS electrolyte. The electrolyte area was 0.935 cm 2 and its thickness 1.54 mm. The cell potenTable 2 Calculated activation energies for the conduction of various cations through NGS-type structures. Mobile ion
Skeleton
Activation energy (kcal/mol)
Ref.
Na+ Na+ Na+, K+ H+(H20)n H+(H20)n
NYS NGS KNGS NGS KNGS
8.58 6.86 8.35 10.98 10.68
[ 15 ] [ 14 ] this work this work this work
tial was maintained constant at 10 V and a current of 10 + 3 mA passed for 100 h. The hydrogen produced was collected over water. The cell eventually ceased to operate due to RTV-cement seal failure. The operating temperature was ~90°C.
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