Journal of The Electrochemical Society, 148 共12兲 A1287-A1293 共2001兲
A1287
0013-4651/2001/148共12兲/A1287/7/$7.00 © The Electrochemical Society, Inc.
The Behavior of Oxygen Transport in Valve-Regulated
Lead-Acid Batteries with Absorptive Glass Mat Separator
Yonglang Guo,a,z Jianyong Wu,a Likun Song,a M. Perrin,b H. Doering,b
and J. Garcheb,*
a
Department of Chemistry, Shandong University, Jinan, 250100 China
Center for Solar Energy and Hydrogen Research, D-89081 Ulm, Germany
b
In the oxygen cycle of valve-regulated lead-acid 共VRLA兲 batteries, there are two ways in which oxygen can move from the
positive to the negative plates, namely, either horizontally to penetrate the absorptive glass mat 共AGM兲 separator, and/or transport
vertically via the gas space. It is found that the oxygen transport depends on the passageway with big void space in the AGM
separator and its rate is proportional to the oxygen partial pressure. The rate constant of vertical transport is about three orders
higher than that of horizontal transport because of the large void space between the AGM separator and plates. However, in the
horizontal direction, the area is very large and the transport path is very short. So the way and the rate of oxygen transport actually
depend on the level of saturation in VRLA batteries. The horizontal transport is dominant when the saturation is less than 93%,
while the vertical transport becomes dominant when it is higher than 93%. The experiments also indicate that with decreasing
saturation, the recombination of more oxygen at the negative plate may oxidize more active Had atoms and therefore, the
overpotential of hydrogen evolution increases obviously.
© 2001 The Electrochemical Society. 关DOI: 10.1149/1.1413990兴 All rights reserved.
Manuscript submitted January 29, 2001; revised manuscript received June 4, 2001. Available electronically October 25, 2001.
In the past years valve-regulated lead-acid 共VRLA兲 batteries
have been widely used in uninterruptible power supplies 共UPS兲,
communications switch-operating in the power generation industry,
electric vehicles, and even as starting, lighting, ignition batteries.1-5
Their existence is allowed by the successful application of the oxygen cycle, as is the case in the nickel-cadmium cell.6-12 During
overcharge, oxygen and hydrogen evolve at the positive and negative electrodes, respectively. And the following processes of oxygen
cycle occur in VRLA batteries13,14
关1兴
Oxygen produced at the positive plates can diffuse through the
electrolyte and transport through the micropores in an absorptive
glass mat 共AGM兲 separator or through the microcracks in a gelled
electrolyte, to come close to the surface of the negative plates. Since
the transport rate of oxygen in the gas phase is much higher than
that in the electrolyte, there are mainly two ways in which oxygen
can move from the positive to the negative plates: by either directly
penetrating the separator from positive to negative plates 共horizontal
transport兲 and/or vertically transporting to the gas space of the battery and reaching the surface of the negative plate vertically again
from the gas space. After that, oxygen is reduced by the electrochemical and/or chemical processes.15,16 The oxygen transport from
positive to negative plates, its penetration through the thin film on
the lead surface, and the surface area available for the reduction may
be rate-controlling.17 The reduction of oxygen depolarizes the negative electrode and therefore inhibits the evolution of hydrogen. In
order to have no premature hydrogen evolution on the negative
* Electrochemical Society Active Member.
z
E-mail: yguo@sdu.edu.cn
plate, the negative/positive material ratio must ensure that the negative plate is still only partially charged when the positive plate is
fully charged.
During the overcharge, the O atoms are formed by the adsorption
of OH radicals and their dehydration at the positive plate, and then,
two O atoms combine to form an oxygen molecule. The rate of
oxygen evolution depends on the OH adsorption and the polarization overpotential.18,19 In the case of the starving electrolyte, the rate
of oxygen evolution is much higher than that of hydrogen evolution,
but its content in the gas space of the battery is much lower than that
of hydrogen.20,21 It means that the fast oxygen cycle occurs in the
VRLA batteries. Until now, much work has been done on the oxygen recombination process.20-28 The experiments indicate that the
rate of the oxygen cycle is closely related to the void space in the
separator, by which oxygen can transport horizontally from the positive plate to the negative one. At the same time, it is found that the
pressure in the gas phase of VRLA batteries increases with charging
and decreases with discharging and during open circuit. So, the vertical oxygen transport occurs undoubtedly. However, we want to
know which process is dominant and what factor influences the rate
of the oxygen transport. In the practical application of VRLA batteries, the gas recombination and material balance are two very important parameters. And the drying out of electrolyte is still one of
the main failure modes of VRLA batteries.1 In this work, the aim is
to further understand the behavior of oxygen transport in VRLA
batteries with AGM separator and its rate-determining step.
Experimental
Cell 1 was composed of one positive plate, one negative plate,
three-layer AGM separators, and a thin pure lead electrode with 1
cm2 surface area as shown in Fig. 1. The plates are of a commercial
12 V/7 Ah VRLA battery with three positive plates and four negative plates. In order to have more oxygen evolved at the positive
plate during overcharging, the positive plate used has the same dimensions as the negative plate, 4.4 ⫻ 6.9 ⫻ 0.24 cm, so that one
is sure that the positive plate is overcharged while the negative plate
is still charging. The AGM separator consisted of fine glass fibers
with 1-3 ␮m diam and was 0.09 cm thick at 20 kPa. The components were assembled in a Plexiglas container, which was connected
with a pressure sensor 共type MPX100D兲, a valve, and a
Hg/Hg2SO4 /H2SO4 共sp gr 1.30 g cm⫺3兲 reference electrode.18 The
electrolyte filled at 100 % saturation was 17.75 g H2SO4 共sp gr 1.30
g cm⫺3兲 and the gas space in cell 1 was 6.28 cm3. Different saturation values can be calculated by water loss after overcharging. The
overpressure of the gas phase in cell 1 and the electrode potentials
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Journal of The Electrochemical Society, 148 共12兲 A1287-A1293 共2001兲
Figure 3. Installation for the measurement of horizontal oxygen transport
through the AGM separator.
Figure 1. Sketch of cell 1, composed of two plates, three AGM separators,
and one pure lead electrode.
were measured by an HP 34970A data acquisition/switch unit connected with a PC computer at 10, 20, 50, and 100 mA overcharge
currents, respectively. It is assumed that oxygen on the small pure
lead electrode from the positive plate can be completely reduced at
⫺1 V, controlled by potentiostat 共HDV-7兲. Therefore, the rate of
oxygen transport through the AGM separator in the horizontal direction was obtained by measuring the reduction current. All experiments were conducted at 25 ⫾ 2°C.
Figure 2 shows a sketch of cell 2, which comprises one positive
plate, one negative plate, and a two-layer AGM separator between
the two plates. The cell was fixed between two Plexiglas plates. And
all sides of the cell were sealed by epoxy resin, except for two vents,
one of which was linked to the pressure sensor while the other was
used for acid filling. In this case, there is almost no gas space in the
cell.
The installation shown in Fig. 3 was used to measure the rate of
the horizontal oxygen transport through the AGM separator. Depending on the experiment series, a one- or two-layer AGM separator was fixed between two Plexiglas plates. The AGM separator was
sealed around with epoxy resin. The oxygen transport area was a
circle with 1 cm diam. First, the separator was flooded with sulfuric
acid. Then the liquid level was lowered and set at different distances
under the circular part of the AGM separator. Since the liquid can be
Figure 2. Sketch of cell 2, composed of two plates and two AGM separators. All around the plates is sealed by epoxy resin.
absorbed in the micropores of the AGM separator, the liquid level
might control the saturation of the circular AGM separator. The rate
of oxygen transport at 8.4 kPa between the opposite sides of the
AGM separator was measured by a self-made foam flowmeter,
which can determine the volume of the oxygen flow per second 共mL
s⫺1兲.
The installation in Fig. 4 was similar to that in a cell, but a
positive plate replaced the negative plate. The back side and edges
of the positive plates were sealed by epoxy resin so that oxygen only
passed through the AGM separator vertically. After the vertical oxygen transport in the separator without the electrolyte was measured,
the plates and the separator were flooded with H2SO4 solution 共sp gr
1.30 g cm⫺3兲 for 40 h, to reach 100 % saturation. Then the expected
saturation was obtained by charging both positive plates for a short
time. During the period, one plate was discharged while the other
plate was charged and the evolving oxygen removed a small amount
of electrolyte from the separator. After this, the voltage between the
two plates was less than 0.2 V so that there was no lead on the plate
that could react with the oxygen. The rate of oxygen transport was
measured by a foam flowmeter at different pressures.
Results and Discussion
At the end of charging in a VRLA battery, the main reactions at
the positive plate are the oxidation of PbSO4 to PbO2, oxygen evolution, and the grid corrosion, and those at the negative plate are the
reduction of PbSO4 to Pb, hydrogen evolution, and oxygen reduction. During overcharging, the reactions on the active mass become
very slow. The corrosion rate of the positive grid is less than 2% of
the charging current.11 And under the balanced conditions, the oxy-
Figure 4. Installation for the measurement of vertical oxygen transport
through the AGM separator.
Journal of The Electrochemical Society, 148 共12兲 A1287-A1293 共2001兲
A1289
Figure 5. Evolution of the overpressure in the gas space of cell 1 at different
charge currents. The charge began at point b and was interrupted at the
maximum 共point d兲 of each curve. Saturation: 94%.
gen cycle in Eq. 1 is completed and the rate of hydrogen evolution
is approximately equal to that of the grid corrosion.12
In cell 1 共Fig. 1兲, since both positive and negative plates have the
same thickness, the capacity of the positive plate is much less than
that of the negative plate. Oxygen evolution is dominant at the positive plate during the overcharging. In order to understand the behavior of the oxygen transport, first, the gas space in cell 1 was filled by
the air after the cell was thoroughly overcharged, then when the
overpressure in the cell reached a minimum 共consumption of all
oxygen, segment ab of Fig. 5兲, the cell was charged at different
currents. After that, the charge was interrupted when the steady oxygen cycle was established. Figure 5 shows the changes in the overpressure in the gas space at different charging currents and 94%
saturation. At the minimum of point b, it is considered that almost
all oxygen in the cell is recombined. When the cell is charged, the
pressure increases first sharply 共segment bc兲 and then steadily 共segment cd兲. However, it drops quickly again during open circuit 共segment de兲. And it increases slowly in the final rest stage 共segment ef兲.
Figure 6 shows the corresponding changes in the potential of the
positive and negative electrodes.
At the beginning of the charging, we observe that the potential of
the positive electrode rises quickly and the polarization overpotential reaches 150-200 mV for the charge currents of 10-100 mA,
while only little polarization appears at the negative plate. Its polarization overpotential changes from 8 to 40 mV. The higher polarization at point c in Fig. 6B is due to the reduction of PbSO4 to Pb and
partial hydrogen formation as long as the negative electrode is not
yet depolarized by oxygen. The increase of the pressure in segment
bc of Fig. 5 is dominated by the oxygen evolution at the positive
plate. As the recombination rate of oxygen becomes increasingly
faster with the increase of the oxygen partial pressure, the overpressure rise in the cell gets slower. In segment cd, the oxygen partial
pressure has already reached a steady state. During this period, the
positive and negative potentials in Fig. 6 are almost unchanged. So,
the evolution of oxygen and hydrogen also reaches a steady process.
This means that a balanced oxygen cycle is established and the
change in the overpressure of segment cd represents the hydrogen
evolution at different charge currents. When the charging is interrupted at point d, the overpressure drops rapidly and reaches a minimum at point e. It indicates that the recombination of oxygen is still
going on in this process. At point e, almost all oxygen in the cell is
reduced. Therefore, the slow increase in pressure in segment ef in
Figure 6. The evolution of the positive 共A兲 and negative 共B兲 electrode potentials obtained from the experiment in Fig. 5. Saturation: 94%.
Fig. 5 represents the hydrogen evolution on open circuit. Since the
overpressure difference between points b and e is very similar to the
growth rate of the overpressure in segment cd, it is considered that
the change in the overpressure is caused by the hydrogen evolution
during polarization. Therefore, according to the hydrogen evolution
during the self-discharge of segment ef and the overpressure difference between points b and e, from point d we can approximately
obtain the oxygen and hydrogen partial pressure. They represent the
oxygen partial pressure in a steady oxygen cycle and the hydrogen
evolution in the polarization at different charge currents, respectively.
To identify the compositions of the gas in cell 1, gas specimens
with 1.5 mL at points b, d, and f of Fig. 5 were taken and analyzed
with a gas chromatograph. An air specimen was also tested. It was
observed that the retention time of the gas specimens and the air in
the chromatographic column of a divinylbenzene copolymer was 16
and 18 s, respectively. Although its precise quantification is very
difficult, it is certain that hydrogen exists in the cell as is proved by
the retention time.
When the charging is interrupted at point d, Fig. 6A shows that
the positive potential drops slowly. This is due to the fact that it is
difficult for the sulfuric acid to diffuse from the micropores in the
positive active mass into bulk solution. But, Fig. 6B shows the polarization of the negative plate is cancelled immediately after open
circuit and an obvious positive polarization appears at point p,
which is related to the ongoing oxygen reduction at a high rate.
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Journal of The Electrochemical Society, 148 共12兲 A1287-A1293 共2001兲
Figure 7. Dependence of the oxygen partial pressure in cell 1 on the charge
current at different saturation.
After that, the potential drops slowly to the open circuit potential of
the negative electrode, which is related to the falling of the oxygen
partial pressure as shown in Fig. 5. The larger polarization at point p
occurs in higher oxygen partial pressure. Therefore, it is clear that
the shift potential in the positive direction represents a mixed potential of the oxygen reduction and the discharge of the negative plate.
Figure 7 shows the dependence of the oxygen partial pressure in
the steady oxygen cycle vs. the charge current at different saturation.
The slope of the straight lines becomes increasingly higher with the
increases of the saturation. The recombination rate of oxygen is
proportional to the oxygen partial pressure in the gas space of the
cell and strongly depends on the saturation. Since the diffusion rate
of oxygen in the gas phase is much faster than that in the liquid, a
little enlargement of the void space in AGM separator would make
the oxygen transport rate faster.
According to the pressure difference between points b and e in
Fig. 5, we can calculate the rate of hydrogen evolution during charging at different charge currents. And from the polarization potential
Figure 8. Dependence of the negative electrode overpotential on the logarithm of the hydrogen evolution rate at different saturation values.
Figure 9. The changes in the oxygen reduction current at the small pure lead
electrode, under the conditions in Fig. 5. The pure lead electrode is polarized
at the potential of ⫺1 V. Area of lead electrode: 1 cm2.
in Fig. 6B, the linear relationship between the negative electrode
overpotential and the logarithm of the hydrogen evolution current is
shown in Fig. 8. It is found that the Tafel slope changes from 28 to
98 mV with the decrease of the saturation from 96.5 to 91.5%. The
changes in the slopes may be influenced by the impurities in the
active mass and the reduction of oxygen. When the straight lines are
extrapolated, the self-discharge rate of one negative plate is from
0.14 to 0.1 mA, which corresponds to the change of the saturation
from 96.5 to 91.5 %. This result is in agreement with the fact that
the self-discharge rate becomes gradually slower during the rest period of the actual battery: the dryer the VRLA battery, the lower its
self-discharge.28 We used the same negative plates to assemble a 12
V/7 Ah VRLA battery. Its average self-discharge rate was 0.103 mA
per negative plate during the rest of four months.
Figure 8 also shows that for the same hydrogen evolution current, the potential of the hydrogen evolution or the negative electrode overpotential increases with the decrease of the saturation. But
during the constant current overcharge of a commercial VRLA battery, the overcharge voltage drops gradually with the consumption
of the electrolyte. Clearly, these apparently contrary results are
closely related to the recombination of oxygen. Since the oxygen
transport becomes more rapid with the water loss, the recombination
of oxygen is accelerated. As a result, the negative electrode is depolarized. And it is possible that more active Had atoms can also be
directly oxidized by oxygen so that the hydrogen evolution is greatly
impeded.
Figure 9 shows the changes in the reduction current on the small
pure lead electrode lying between both AGM separators. It is found
that the reduction current reaches a steady value in a very short time
and this value increases with the increase of the charge current.
However, the reduction current drops slowly during the open circuit
period, which corresponds to the change of the overpressure in Fig.
5 共segment de兲. Since the small pure lead electrode is fully charged
before the experiments and the hydrogen evolution is very small at
⫺1 V on it, the reduction currents shown in Fig. 9 are the reduction
of oxygen coming from the positive plate in the horizontal direction.
So, according to the area of the plate and that of the small lead
electrode, the horizontal oxygen transport current can be calculated
and is shown in Fig. 10A at different saturation values. It indicates
that the horizontal oxygen transport current strongly depends on the
saturation: the lower the level of saturation, the faster the rate of
horizontal oxygen transport. And a high charge current makes the
Journal of The Electrochemical Society, 148 共12兲 A1287-A1293 共2001兲
A1291
Figure 11. Changes in the overpressure in cell 2 during the charging and
open circuit at point d. Saturation: 93%.
Figure 10. Oxygen transport current in the horizontal direction 共A兲 and ratio
of horizontal to vertical oxygen transport rates 共B兲 at different saturation
values in cell 1.
oxygen partial pressure in the gas space increase so that the horizontal oxygen transport becomes faster.
In the oxygen cycle, oxygen can directly pass through the AGM
separator from the positive to negative plates and/or it can also go
into the gas space and then reach the surface of the negative plate
vertically. So according to the charge current and the rate of the
horizontal transport in Fig. 10A in a steady oxygen cycle, we can
obtain the ratio of the horizontal to vertical oxygen transport at
different saturation values; they are shown in Fig. 10B. It is interesting to find that the horizontal oxygen transport is dominant when
the saturation is less than 93%, while the vertical oxygen transport
becomes dominant when it is higher than 93%. At the same time, the
rate of the vertical oxygen transport becomes quicker with the increase of the charge current. Therefore, two ways of oxygen transport exist in the oxygen cycle in an AGM VRLA cell and their
transport resistance depends on the level of saturation in the AGM
separator.
Since cell 2 in Fig. 2 is sealed all around, there is almost no gas
space in the cell and then no vertical oxygen transport occurs. Oxygen evolving from the positive plate only transports horizontally
through the AGM separator. Figure 11 shows the changes in the
overpressure in cell 2. It is similar to Fig. 5. But the overpressure
grows very rapidly. This is because little gas space exists in the cell
and only a little hydrogen evolution can make the overpressure rise
quickly. Furthermore, the initial composition in the void space, especially hydrogen, can also influence the change of the overpressure. In the case of cell 2, it is very difficult for the gas in the
micropores to exchange with the air outside. So the change in the
overpressure in Fig. 11 is not completely proportional to the charge
current. When the charge is interrupted at point d, the overpressure
drops quickly and the oxygen partial pressure can be calculated as in
Fig. 7. Figure 12 shows the changes in the oxygen partial pressure at
93% saturation and different currents. For the purpose of comparison, the data of cell 1 in Fig. 7 at 91.5 and 94% saturation are also
shown. It is interesting to find that the oxygen partial pressure in cell
2 almost rises linearly with the increase of the charge current as
found already for cell 1. Even if the level of the saturation for cell 2
lies between 91.5 and 94% for cell 1, the oxygen partial pressure in
cell 2 is higher than that of cell 1. Similar results can also be obtained at other saturation values of cell 2. This indicates that if the
vertical transport way for oxygen is prevented, a higher oxygen
partial pressure is needed to accelerate the oxygen transport in the
horizontal direction in order to achieve the steady state conditions.
Figure 12. Dependence of the oxygen partial pressure on the charge current
in different cells.
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Journal of The Electrochemical Society, 148 共12兲 A1287-A1293 共2001兲
Figure 13. Horizontal oxygen transport rate at different liquid levels and 8.4
kPa between the opposite sides of the AGM separator in the installation of
Fig. 3.
In order to study the rate of oxygen transport through the AGM
separator, Fig. 3 shows the installation by which the oxygen flow in
the horizontal direction can be directly measured at different pressures. In this case, oxygen only passes the circular part of the AGM
separator. Different saturation levels in the separator are controlled
by the liquid level. The lowest pressure difference between the opposite sides of the AGM separator was 6 kPa. No oxygen flow can
be observed unless the pressure difference exceeds this value. It
means that it needs an additional pressure to overcome the capillary
action of the liquid. Figure 13 shows the rate of the horizontal oxygen transport through the AGM separator at different liquid levels. It
indicates that with the decrease of the liquid level, the rate of oxygen transport is almost unchanged until the height of the liquid level
reaches 15 cm. We choose the pressure difference of 8.4 kPa between the opposite sides of the AGM separator because a steady
oxygen flow could be obtained at this pressure. When the pressure
difference increases, some liquid will be removed and more micropores appear in the AGM separator so that the oxygen transport
rate becomes faster and faster. If a two-layer AGM separator was
fixed in the installation of Fig. 3, it is interesting to find that oxygen
not only flows through the circular AGM separator but also escapes
from the AGM separator underneath the liquid. This phenomenon
cannot be observed in the experiment with one-layer AGM separator. It is clear that oxygen can easily transport through the big micropores in the vertical direction because there is a large void space
between the two AGM separators. Therefore, the rate of oxygen
transport strongly depends on the dimension of the micropores and
the pressure difference. And the liquid level does not influence the
oxygen transport very much.
Figure 14 shows the changes in the vertical oxygen transport
at different pressures and 0, 85, 92, and 96% saturation. It is
found that the rate of the vertical oxygen transport rises linearly
with the increase of the pressure difference between upper side
and lower side of the AGM separator. And the lower the level
of saturation is, the faster the rate of the oxygen transport. Their
slopes are 1.33 ⫻ 10⫺5 , 3.26 ⫻ 10⫺6 , 1.33 ⫻ 10⫺6 , and 7.07
⫻ 10⫺7 mol s⫺1 kPa⫺1 at 0, 85, 92, and 96% saturation, respectively. The falling slope is approximately proportional to the reduction of void space or the increase of the saturation in the separator.
It indicates that the rate of oxygen transport is closely related to the
level of saturation.
The rate of the oxygen transport through the AGM separator is
proportional to the pressure, p, and the area of the separator, A, and
is inversely proportional to its thickness, l. Therefore, the rate of the
oxygen transport can be expressed by
Figure 14. Dependence of the vertical oxygen transport rate on the pressure
difference between the opposite sides of the AGM separator at 0, 85, 92, and
96% saturation in the installation of Fig. 4.
Ap
dn
⫽k
dt
l
关2兴
where n is the molar number of oxygen and k is the rate constant
of the oxygen transport. For the horizontal oxygen transport
in Fig. 3 with one-layer AGM separator, the rate constant
can be calculated from the data in Fig. 13, and is 7.9
⫻ 10⫺9 mol 共s kPa cm兲⫺1, while from the slopes in Fig. 14, the
vertical rate constants are 8.5 ⫻ 10⫺5 , 2.1 ⫻ 10⫺5 , 8.5 ⫻ 10⫺6 ,
and 4.5 ⫻ 10⫺6 mol共s kPa cm兲⫺1 at 0, 85, 92, and 96% saturation,
respectively. The results show that the rate constant of the oxygen
transport in the horizontal direction is about three orders lower than
that in the vertical direction. But in the horizontal transport, the
transport area is very large and its path is very short. For the plates
with the dimension of 4.4 ⫻ 6.9 cm and the AGM separator of 0.09
cm thickness, the value of A/l in Eq. 2 in the horizontal transport is
also about three orders higher than that in the vertical transport.
Thus, the oxygen transport rates in both directions are in the same
order. From the difference between two rate constants, we can see
clearly that oxygen easily passes through the void space between the
plate and the separator. Since the electrolyte prefers to fill the small
micropores in the AGM separator under the capillary action, the gas
only occupies the large micropores. Therefore, the void space between the plate and the separator seems to be much larger than the
micropores in the AGM separator. It indicates that the dimension of
the fiber and its structure will affect the rate of the horizontal oxygen
transport through the AGM separator. In the same way, the initial
compression rate applied on the plate stack as well as the ability of
the separator to keep the compression along cycling will influence
the transport rate of oxygen a lot and therefore, the state of charge of
the negative plate and the thermal processes in a VRLA battery with
AGM separator.
Conclusions
During the overcharge of a VRLA battery, a balanced oxygen
cycle is established. There are mainly two ways in which oxygen
can transport from the positive to negative plates: either it directly
penetrates the AGM separator 共horizontal way兲 or it transports vertically via the gas space of the battery. The transport rates and its
passageway will significantly influence the recombination of oxygen
and therefore the float life of the battery.
When the polarization overpotential of the positive electrode
rises obviously during charging, the changes in the gas pressure in
Journal of The Electrochemical Society, 148 共12兲 A1287-A1293 共2001兲
the battery are mainly caused by the oxygen evolution. With the
increase of the oxygen partial pressure, the rate of oxygen transport
and its recombination become faster and faster. Consequently, all
oxygen produced at the positive plate transports to the negative plate
and is reduced. In this case, the hydrogen evolution during the
charging and open circuit is also visible and its partial pressure
increases slowly.
Since the oxygen transport rate in a gas phase is much higher
than that in a liquid, the oxygen transport in a cell strongly depends
on the level of saturation. The horizontal oxygen transport is dominant when the level of saturation is less than 93%, while the vertical
oxygen transport rate becomes dominant when it is higher than
93%. With the increase of the saturation, the resistance to oxygen
transport becomes increasingly greater so that most oxygen has to
transport vertically via the gas space.
The oxygen transport rate is also closely related to the dimension
of the micropores in the AGM separator. The electrolyte prefers to
fill the small micropores because of the capillary action, while the
gas remains in the large void space. Since the void space between
the separator and the plates is much larger than the micropores in the
AGM separator, the rate constant of oxygen in the vertical direction
is about three orders higher than that in the horizontal direction. But
the ratio of the area to the thickness in the horizontal transport is
inversely about three orders higher than that in the vertical transport.
Therefore, both transport rates are of the same order. And the oxygen transport path and its rate strongly depends on the design of a
battery and the structure of the AGM separator. If a few coarse fibers
are mixed in the glass fiber, large micropores can exist in the AGM
separator, which may make the oxygen transport become more
rapid.
During the overcharge and open circuit of a VRLA battery, the
hydrogen evolution rate is visible. Since it is very difficult for hydrogen to be oxidized at a positive plate, the hydrogen may escape
through the valve and even through the battery wall, which results in
the drying out of the battery. When a battery is overcharged at a
constant voltage or a constant current, the hydrogen evolution rate is
related to the level of saturation. The lower the saturation is, the
higher the overpotential of the hydrogen evolution becomes. This is
because the low saturation promotes the oxygen cycle in a VRLA
battery, which makes the overpotential of the negative electrode decrease obviously. Furthermore, the reduction of more oxygen at the
negative plate may oxidize more active Had atoms so that the hydrogen evolution is greatly impeded.
A1293
Acknowledgment
The authors are grateful to DFG in Germany and NSFC in China
for the financial support for this work.
The Center for Solar Energy and Hydrogen Research assisted in meeting
the publication costs of this article.
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