Electrochemical behavior of hydride ion in a LiBr-KBr-CsBr eutectic melt

Article abstract of DOI:10.1149/1.1591755

The electrochemical behavior of hydride ion at a molybdenum electrode in a LiBr-KBr-CsBr-LiH melt has been studied by cyclic voltammetry and chronopotentiometry. The obtained voltammograms and chronopotentiograms were classified as a reversible system. The electron number was found to be one. It was found that hydride ion was anodically oxidized at around 0.50 V vs. Li⁺/Li and pure hydrogen gas was generated according to the following reaction H^- → 1/2H₂ + e. Using voltammetry and chronopotentiometry diffusion coefficients of hydride ion were estimated as 1.2 × 10^-1 exp(-4.5 × 10⁴/RT) cm² s^-1 at 523-673 K.

Full text of DOI:10.1149/1.1591755

Journal of The Electrochemical Society, 150 ͑8͒ E403-E408 ͑2003͒
E403
0013-4651/2003/150͑8͒/E403/6/$7.00 © The Electrochemical Society, Inc.
Electrochemical Behavior of Hydride Ion in a LiBr-KBr-CsBr
Eutectic Melt
*
**,z
Takeo Kasajima, Tokujiro Nishikiori, Toshiyuki Nohira, and Yasuhiko Ito
Department of Fundamental Energy Science, Graduate School of Energy Science, Kyoto University,
Sakyo-ku, Kyoto 606-8501, Japan
The electrochemical behavior of hydride ion at a molybdenum electrode in a LiBr-KBr-CsBr-LiH melt has been studied by cyclic
voltammetry and chronopotentiometry. The obtained voltammograms and chronopotentiograms were classified as a reversible
system. The electron number was found to be one. It was found that hydride ion was anodically oxidized at around 0.50 V vs.
Liϩ/Li and pure hydrogen gas was generated according to the following reaction HϪ → 1/2H2 ϩ e. Using voltammetry and
chronopotentiometry diffusion coefficients of hydride ion were estimated as 1.2 ϫ 10Ϫ1 exp(Ϫ4.5 ϫ 104/RT) cm2 sϪ1 at
523-673 K.
© 2003 The Electrochemical Society. ͓DOI: 10.1149/1.1591755͔ All rights reserved.
Manuscript submitted September 27, 2002; revised manuscript received February 24, 2003. Available electronically July 1, 2003.
It has been known for many years that hydride (HϪ) ion is pro-
duced in molten alkali metal halides by dissolution of a saline hy-
dride like LiH.1 Electrochemical reactions involving hydride ion in
molten salts are worth studying due to their promising applications
in the energy-related field. For instance, the electrochemical reac-
tions of M-H/HϪ systems provide a convenient and precise means to
investigate hydrogen behavior in metals at high temperatures.2-5 Es-
pecially, we have succeeded in investigating Pd-Li-H3 and Ti-H
systems4-5 at 673-773 K using the LiCl-KCl-LiH melt as an electro-
lyte. Also a Li-H2 thermally regenerative fuel cell can be con-
structed using the reaction of the H2 /HϪ system.6-7 Since the Li-H2
thermally regenerative fuel cell converts heat to electricity with high
efficiency, it seems to be a promising method for efficient utilization
of high temperature heat. As fundamental data to develop these ap-
plications, we have already studied on the electrochemical behavior
of hydride ion in a LiCl-KCl eutectic melt ͑mp 625 K͒ at 673-740
K8-9 and in a LiF-KF-NaF eutectic melt ͑mp 727 K͒ at 773 K.10
However, operation temperatures have been limited to high tempera-
tures because of high melting points of these melts. The above-
mentioned applications have further possibilities if operating tem-
Nikkato, SSA-S͒ and continued to dehydrate in vacuum by heating
at 423 K for at least 24 h. LiH ͑99.0ϩ mol %,12 Wako Pure Chemi-
cal Industries, Ltd.͒ was used as a hydride ion source.8-10 Chromel-
alumel thermocouple was used for temperature measurement. Tem-
perature was kept within 1 K by using a temperature control unit
͑DSM-2; Shimaden Co., Ltd.͒. A molybdenum plate ͑5 ϫ 4 ϫ 0.2
mm, 99.95%, The Nilaco Corp.͒ was used as the working electrode
with a glassy carbon rod ͑50 ϫ ␾ 3 mm, Tokai Carbon Co., Ltd.͒ as
the counter electrode. The reference electrode was an Al-Li alloy in
the ͑␣ ϩ ␤͒ coexisting phase state prepared electrochemically from
an aluminum wire ͑4 ϫ ␾ 1 mm, 99.99%, The Nilaco Corp.͒. The
equilibrium potential of this reference electrode is thought to be
determined by the following reversible reaction13-15
Al ␣͒ ϩ Liϩ ϩ e ꢀ AlLi ␤͒
͓1͔
͑
͑
The potential of this electrode was calibrated with that of the Liϩ/Li
dynamic reference electrode which was prepared by electrodeposit-
ing lithium metal at a nickel wire ͑4 ϫ ␾ 1 mm, 99.99%, The Nilaco
Corp.͒.11 All potentials in this paper were referred to this Liϩ/Li
potential.
Electrochemical measurement was conducted by a potentio/
galvanostat ͑HZ-3000; Hokuto Denko Co., Ltd.͒ controlled by a
personal computer ͑FMVS31673; Fujitsu͒.
peratures are lowered to around 500-600
K ͑medium-range
temperatures͒. As to utilization of metal-hydrogen ͑M-H͒ systems,
efficient use of the industrial waste heat at medium-range tempera-
tures is expected because the hydrogen absorption/desorption reac-
tion of some M-H systems proceeds with large heat exchange. Also
concerning the Li-H2 thermally regenerative fuel cell, it has been
shown from thermodynamic consideration that theoretical efficiency
becomes higher at lower operating temperatures. Recently, we found
that the LiBr-KBr-CsBr eutectic melt is a promising electrolyte at
medium-range temperatures ͑mp about 498 K͒.11 The aim of this
study is to reveal the electrochemical behavior of hydride ion in the
LiBr-KBr-CsBr eutectic melt, which is very important as fundamen-
tal information for the above-mentioned applications. Electrochemi-
cal reversibility, electron numbers of the anodic reaction, and diffu-
sion coefficients of hydride ion were investigated by cyclic
voltammetry ͑CV͒ and chronopotentiometry at 523-673 K.
Results and Discussion
Cyclic voltammetry.—Cyclic voltammetry was carried out at a
molybdenum electrode at various scan rates in the LiBr-KBr-CsBr
eutectic melt to which 2.0 mol % LiH was added. Ohmic drops were
compensated after each measurement by using the measured solu-
tion resistances, 1.7 ⍀. The voltammograms obtained at 573 K are
shown in Fig. 2. An anodic peak at around 0.7 V and a correspond-
ing cathodic peak are observed at all scan rates. According to our
previous result,11 the anodic currents have been confirmed to corre-
spond to hydrogen gas evolution. The reaction formula is estimated
as
HϪ → 1/2H2 ϩ e
͓2͔
Experimental
The apparatus for electrochemical measurement is illustrated in
Fig. 1. All experiments were conducted in a glove box with an argon
atmosphere. The LiBr-KBr-CsBr eutectic ͑LiBr:KBr:CsBr
ϭ 56.1:18.9:25.0 mol %, reagent grade, LiBr; 99ϩ wt %, Aldrich
Chemical Company, Inc., KBr; 99.0ϩ wt %, Wako Pure Chemical
Industries, Ltd., and CsBr; 99 wt %, Mitsuwa Chemicals Co., Ltd.͒
was contained in a high purity alumina crucible ͑99.5%, Al2O3 ;
In order to further investigate the reaction, cyclic voltammetry
was conducted at different concentrations of added LiH. Figure 3
shows the variation of anodic peak current density, ipa , with the
square root of potential scan rate for various amounts of added LiH
at 573 K. A linear dependence of ipa vs. the square root of potential
scan rate is observed for all cases. Figure 4 shows the plot of ipa
with the amounts of added LiH. Linear relationship is also obtained.
The above results indicate that the anodic reaction of hydride ion is
the reversible electrode process. In Fig. 4, the extrapolations of an-
odic current densities to zero converge at around 0.25 mol %. This
result can be explained; 0.25 mol % of LiH were consumed by
* Electrochemical Society Active Member.
** Electrochemical Society Fellow.
z E-mail: y-ito@energy.kyoto-u.ac.jp
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Journal of The Electrochemical Society, 150 ͑8͒ E403-E408 ͑2003͒
Figure 3. Variations of anodic peak current density, ipa , with square root of
potential scan rate, ␯1/2, for various amounts of added LiH at 573 K.
is the Faraday constant. Using this equation, n is calculated to be
1.0Ϯ0.1 at 573 K, which is consistent with Reaction 2.
The Epa/2 value can be used to calculate the voltammetric half-
wave potential, E1/2 , which is given by16
Figure 1. Schematic representation of the experimental apparatus for elec-
trochemical measurement; ͑1͒ thermocouple, ͑2͒ working electrode ͑molyb-
denum plate͒, ͑3͒ counter electrode ͑glassy carbon rod͒, ͑4͒ Al-Li reference
electrode, ͑5͒ Liϩ/Li electrode ͑nickel wire͒, ͑6͒ stainless holder, ͑7͒ alumina
crucible, and ͑8͒ LiBr-KBr-CsBr eutectic melt.
RT
E
1/2 ϭ Epa/2 ϩ 1.09
͓4͔
nF
reaction with contaminants such as residual water or heavy metal
ions in the melt. Therefore, it is reasonable to regard 0.25 mol % as
the zero point of the concentration of hydride ion.
Using this equation and the values of Epa/2 obtained from Fig. 2, E1/2
is calculated to be 0.70 Ϯ 0.01 V at 573 K. The E1/2 is in turn related
For a reversible anodic process, the following equation is avail-
able to calculate an electron number, n, of the reaction16
16
to the formal potential, E0 , as
Ј
D1O/2
RT
RT
nF
1/2 ϭ E0
Ϫ
ln
͓5͔
͉
Epa Ϫ Epa/2
͉
ϭ 2.2
͓3͔
Ј
E
nF
D1H/2
Ϫ
where Epa denotes the anodic peak potential, Epa/2 is the anodic
half-peak potential, R is the gas constant, T is the temperature, and F
Figure 2. Cyclic voltammograms at a molybdenum electrode in a LiBr-KBr-
CsBr-LiH ͑2.0 mol % LiH added͒ melt at 573 K. Scan rates are ͑a͒ 2.0, ͑b͒
Figure 4. Variations of anodic peak current density, ipa , with the amounts of
added LiH for various potential scan rates at 573 K.
1.0, ͑c͒ 0.5, and ͑d͒ 0.2 V sϪ1
.
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Journal of The Electrochemical Society, 150 ͑8͒ E403-E408 ͑2003͒
E405
Figure 5. Convoluted cyclic voltammogram for an anodic wave in a LiBr-
KBr-CsBr-LiH ͑1.0 mol % LiH added͒ melt at 573 K. Scan rate is 1.0 V sϪ1
.
Figure 6. Logarithmic analysis of the convoluted voltammogram at a mo-
lybdenum electrode in a LiBr-KBr-CsBr-LiH ͑1.0 mol % LiH added͒ melt at
573 K. Scan rate is 1.0 V sϪ1
.
where DH is the diffusion coefficient of hydride ion and DO is the
Ϫ
diffusion coefficient of the species anodically oxidized from hydride
ion in the melt.
Chronopotentiometry.—Chronopotentiometry was conducted to
further investigate the anodic reaction of hydride ion in the melt.
Figure 7 shows a typical chronopotentiogram obtained at a molyb-
denum electrode at an applied anodic current density of 700 mA
cmϪ2. Considering the result of CV, the observed potential plateau is
thought to be due to the anodic oxidation of hydride ion. The tran-
sient time, ␶, is determined by the duration time of the potential
plateau. The variation of the inverse square root of ␶ against the
applied current densities for different concentrations of LiH added is
shown in Fig. 8, in which linear relationships are observed. Figure 9
shows that the square root of ␶ also increases linearly with increas-
ing amounts of added LiH for various applied current densities. The
above results also support that the anodic reaction of hydride ion is
the reversible electrode process.
For a reversible anodic wave, ipa is given by Eq. 6 when both the
reactant and the product are soluble16
1/2
nF
ipa ϭ 0.4463nFC
␯
1/2D1H/2
͓6͔
Ϫ
ͩ ͪ
RT
where C is the bulk concentration of hydride ion and ␯ is the scan
rate. Then, DH
Ϫ
is calculated by this equation as (8.0 Ϯ 0.7)
ϫ 10Ϫ6 cm2 sϪ1 at 573 K.
For further analysis, a convolution technique was applied to this
result. The convoluted current, I(t), is calculated as the following
convolutive transformation of the observed current, j(t)16
t
1
j u͒
͑
As shown in Fig. 9, the extrapolations of the square root of ␶ to
zero at all anodic current densities converge at around 0.25 mol %,
in agreement with the voltammetric investigation ͑Fig. 4͒.
I t͒ ϭ
1/2 du
͓7͔
͑
͵
1/2
␲
t Ϫ u͒
͑
0
Figure 5 shows the convoluted cyclic voltammogram for the anodic
wave in the LiBr-KBr-CsBr-LiH ͑1.0 mol % LiH added͒ melt at 573
K. For comparison, a theoretical reversible curve is also shown in
Fig. 5. Since the experimental curve almost overlaps with the theo-
retical curve, this electrochemical reaction is regarded as a revers-
ible system. For a reversible wave, the following relation is valid17
RT I1 Ϫ I
E ϭ E1/2 Ϫ
ln
͓8͔
nF
I
where I1 is a limiting value of the convoluted current. The plot of
ln (I Ϫ I)/I against E is shown in Fig. 6. The relation between E
͓
͔
1
and ln (I Ϫ I)/I shows a linear relationship, which confirms that
͓
͔
1
the oxidation of hydride ion is reversible. The obtained E1/2 value
from this plot is 0.71 V at 573 K, which is in accordance with the
value estimated by using Eq. 4. From the slope of RT/nF the n
value is estimated to be 1.2 at 573 K. Moreover, the diffusion coef-
ficient of hydride ion is obtained by the following relation16
I1 ϭ nFACD1/2
͓9͔
Ϫ
H
where A is the electrode area. From this equation, DH
Ϫ
is estimated
Figure 7. Typical chronopotentiogram at a molybdenum electrode in a LiBr-
KBr-CsBr-LiH ͑2.0 mol % LiH added͒ melt at 573 K. Applied current den-
as 7.4 ϫ 10Ϫ6 cm2 sϪ1 at 573 K. This value is very close to that
obtained from Eq. 6.
sity is 700 mA cmϪ2
.
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E406
Journal of The Electrochemical Society, 150 ͑8͒ E403-E408 ͑2003͒
Figure 10. Plot of the measured potential against ln (␶1/2 Ϫ t1/2)/t1/2 for
chronopotentiogram at a molybdenum electrode in a LiBr-KBr-CsBr-LiH
͓
͔
Figure 8. Variations of inverse square root of transient time with applied
current densities for various amounts of added LiH at 573 K.
͑2.0 mol % LiH added͒ melt at 573 K. Applied current density is 700 mA
cmϪ2
.
A plot of E against ln (␶1/2 Ϫ t1/2)/t1/2 for the chronopotentio-
gram in Fig. 7 is shown in Fig. 10. The plot shows a good linear
relationship, which shows that the reaction is highly reversible, and
the following relation is valid16
͓
͔
D1O/2
RT
nF
␶/4 ϭ E0
Ϫ
ln
͓11͔
Ј
E
D1H/2
Ϫ
RT
nF
␶
1/2 Ϫ t1/2
t1/2
Comparing Eq. 5 with Eq. 11, E␶/4 should be equal to the E1/2 . The
E ϭ E␶/4 Ϫ
ln
͓10͔
obtained E is nearly equal to E1/2 estimated from the voltammo-
␶/4
gram.
For the reversible reaction, the applied current density, i, is given
by the Sand equation16
where E is called the quarter-wave potential. From the slope of
␶/4
the straight line in this plot, the electron number is estimated as 1.0
Ϯ 0.1 at 573 K, which almost equals to the value obtained in the
previous section. E␶/4 is directly obtained as 0.70 Ϯ 0.02 V from the
nFD1H/2
␲
1/2
1/2
i␶
Ϫ
ϭ
͓12͔
potential value at ln (␶1/2 Ϫ t1/2)/t1/2 ϭ 0. E is related to E0
Ј
C
2
͓
͔
␶/4
as16
Figure 11. Cyclic voltammograms at a molybdenum electrode in a LiBr-
KBr-CsBr-LiH ͑2.0 mol % LiH added͒ melt at 673 K. Scan rates are ͑a͒ 2.0,
Figure 9. Variations of square root of transient time with the amounts of
added LiH for various applied current densities at 573 K.
͑b͒ 1.0, ͑c͒ 0.5, and ͑d͒ 0.2 V sϪ1
.
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Journal of The Electrochemical Society, 150 ͑8͒ E403-E408 ͑2003͒
E407
Figure 12. Cyclic voltammograms at a molybdenum electrode in a LiBr-
KBr-CsBr-LiH ͑2.0 mol % LiH added͒ melt at 523 K. Scan rates are ͑a͒ 2.0,
͑b͒ 1.0, ͑c͒ 0.5, and ͑d͒ 0.2 V sϪ1
.
Figure 13. Arrhenius plot of DHϪ.
DH
ϭ D exp ϪE /RT͒ ϭ 1.2 ϫ 10Ϫ1 exp Ϫ4.5 ϫ 104/RT͒
͑ ͑
0 a
Ϫ
According to this equation, DH
Ϫ
is estimated to be 1.1(Ϯ0.1)
ϫ 10Ϫ5 cm2 sϪ1 at 573 K. This value agrees well with that obtained
͓13͔
by CV.
where D0 is the pre-exponential factor and Ea is the activation en-
ergy. The reported DH
Ϫ
in the LiCl-KCl-LiH melt at 673-740 K is
ϭ 4.4 ϫ 10Ϫ2 exp Ϫ2.8 ϫ 104/RT͒
DH ͓14͔
͑
represented by8
Effect of temperature on the electrochemical behavior of hydride
ion.—CV was conducted also at 673 and 523 K. The obtained vol-
tammograms at 673 and 523 K are shown in Fig. 11 and 12, respec-
tively. Compared with the voltammograms at 573 K, current densi-
ties are larger at 673 K and smaller at 523 K. When ipa were plotted
with the square root of potential scan rate or the amounts of added
LiH, similar linear tendencies were also obtained at 673 K and 523
K. However, slight positive shifts of anodic peak potential are ob-
served with increase of scan rate. The shift becomes larger as ex-
perimental temperature decreases. This is explained by that the com-
pensation of ohmic drop is not appropriate enough and/or the
reaction is not completely reversible at lower temperatures. Assum-
Ϫ
The estimated D0 and Ea in Eq. 13 are a little larger than the re-
ported values. This result suggests that the interaction between cat-
ions and anions including hydride ion is different for these melts.
However, there is only one report which can be compared with the
present results. Therefore, it is very interesting to study other alkali
halide-LiH melts, such as LiF-KF-CsF-LiH, LiCl-KCl-CsCl-LiH,
and LiI-KI-CsI-LiH melt.
Conclusion
ing a reversible reaction, however, n, E1/2 , and DH were calculated
Ϫ
at 673 and 523 K from the obtained voltammograms and the con-
voluted voltammograms. Chronopotentiometry was also conducted
at 673 and 523 K. Because the reaction was regarded as reversible at
The electrochemical behavior of hydride ion at a molybdenum
electrode in the LiBr-KBr-CsBr-LiH melt was studied by means of
CV and chronopotentiometry at 523-673 K. It is confirmed that the
anodic oxidation reaction of hydride ion is a reversible process. The
electron number is obtained nearly 1 in good agreement with each
analysis. Diffusion coefficients of hydride ion in this melt were also
estimated from the obtained voltammograms and chronopotentio-
each temperature, n, E␶/4 , and DH
marized in Table I. n was calculated to be about 1 at all tempera-
tures, which is consistent with Reaction 2. E1/2 ͑or E␶/4) and DH
values were estimated by using three different methods in accor-
dance with each other at each temperature. The DH value was
smaller as experimental temperature decreased. Figure 13 shows an
Arrhenius plot of DH , in which the logarithm of DH is plotted
is ex-
Ϫ
values were calculated, as sum-
Ϫ
grams as DH
Ϫ
ϭ 1.2 ϫ 10Ϫ1 exp(Ϫ4.5 ϫ 104/RT) cm2 sϪ1
.
Ϫ
Acknowledgments
Ϫ
Ϫ
against the inverse of temperature. From this result, DH
Ϫ
This study was supported by Industrial Technology Research
Grant Program in 2002 from the New Energy and Industrial Tech-
pressed as
Table I. The electron number of the reaction, n, the voltammetric half wave potential, E1Õ2 , the quarter-wave potential, E␶Õ4 , and the diffusion
coefficient of hydride ion, DHÀ, in a LiBr-KBr-CsBr eutectic melt obtained by cyclic voltammetry, convolution method, and chronopotentiom-
etry.
n
E1/2 or E␶/4 /V vs. Liϩ/Li
DHϪ/10Ϫ5 cm2 sϪ1
T/K
CV
Convolution
CP
CV
Convolution
CP
CV
Convolution
CP
673
573
523
1.2
1.2
1.1
0.67
0.71
0.73
3.4
0.74
0.39
1.0 Ϯ 0.2
1.0 Ϯ 0.1
1.1 Ϯ 0.2
0.94 Ϯ 0.03
1.0 Ϯ 0.1
1.1 Ϯ 0.1
0.65 Ϯ 0.04
0.70 Ϯ 0.01
0.71 Ϯ 0.01
0.66 Ϯ 0.01
0.70 Ϯ 0.02
0.76 Ϯ 0.02
3.2 Ϯ 0.2
0.80 Ϯ 0.07
0.30 Ϯ 0.04
3.9 Ϯ 0.2
1.1 Ϯ 0.1
0.35 Ϯ 0.05
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Journal of The Electrochemical Society, 150 ͑8͒ E403-E408 ͑2003͒
n
R
T
Number of electrons
nology Development Organization ͑NEDO͒ of Japan. It was also
supported by a Grant-in-Aid for 21st COE program ͑Establishment
of COE on Sustainable-Energy System͒ from the Ministry of Edu-
cation, Culture, Sports, Science and Technology. One of the authors,
T. Nishikiori, thanks JSPS Research Fellowships for Young Scien-
tists for the financial support.
Gas constant, 8.314 J KϪ1 molϪ1
Temperature, K
Greek
␯
␶
Scan rate in voltammetry, V sϪ1
Transition time in chronopotentiometry, s
Kyoto University assisted in meeting the publication costs of this article.
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List of Symbols
A
C
Electrode area, cm2
Bulk concentration of hydride ion, mol cmϪ3
D0 Pre-exponential factor
DH
Ϫ
Diffusion coefficient of hydride ion, cm2 sϪ1
DO Diffusion coefficient of the species anodically oxidized from hydride ion, cm2
sϪ1
E0
Formal potential, V vs. Liϩ/Li
Ea Activation energy of diffusion of hydride ion in a LiBr-KBr-CsBr-LiH melt, J
Ј
molϪ1 KϪ1
Voltammetric half-wave potential in voltammetry, V vs. Liϩ/Li
Epa/2 Anodic half-peak potential in voltammetry, V vs. Liϩ/Li
E1/2
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126, 2258 ͑1979͒.
Anodic peak potential in voltammetry, V vs. Liϩ/Li
Epa
Quarter-wave potential in chronopotentiometry, V vs. Liϩ/Li
E
␶/4
F
The Faraday constant, 96485 C molϪ1
I1 Limiting value of convoluted current in voltammetry, C sϪ1/2
ipa Anodic peak current density in voltammetry, A cmϪ2
I(t) Convoluted current in voltammetry, C sϪ1/2
16. A. J. Bard and L. R. Faulkner, Electrochemical Methods: Fundamentals and Ap-
plications, John Wiley & Sons, New York ͑1980͒.
ˇ
i
Current density in the Sand equation, A cmϪ2
17. J. C. Imbeaux and J. M. Saveant, J. Electroanal. Chem. Interfacial Electrochem.,
j(t) Current in the convolution method to the voltammograms, A
44, 169 ͑1973͒.
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