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Langmuir 2009, 25, 542-547
Can Water Store Charge?
Kate Ovchinnikova and Gerald H. Pollack*
Department of Bioengineering, Box 355061, UniVersity of Washington, Seattle, Washington 98195
ReceiVed July 28, 2008. ReVised Manuscript ReceiVed September 25, 2008
Previous work from this and other laboratories has demonstrated large pH gradients in water. Established by passing
current between immersed electrodes, pH gradients between electrodes were found to disappear slowly, persisting
for tens of minutes after the current had been turned off. We find here that these pH gradients reflect a genuine
separation of charge: at times well after disconnection of the power supply, current could be drawn through a resistor
placed between the charging electrodes or between pairs of electrodes positioned on either side of the midline between
original electrodes. In some experiments, it was possible to recover the majority of charge that had been imparted
to the water. It appears, then, that water has the capacity to store and release substantial amounts of charge.
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Introduction
Electrolytic processes in pure water are commonly ascribed
to the following sequence of events: current flow, electrode
polarization, interfacial double-layer formation around the
electrodes, and establishment of equilibrium.1 The charge carriers
responsible for this sequence include negatively charged OH
groups, protons, hydronium ions, and perhaps larger scale charged
clusters.2
In a previous report from this laboratory,3 it was found that
the distribution of charge carriers did not fit this conventional
picture. During the passage of current through a chamber filled
with water and pH-sensitive dye, large pH gradients were created.
The region next to the anode developed extremely low pH, while
the region next to the cathode developed extremely high pH.
Each of these zones of approximately uniform color spread
throughout roughly one-half the chamber, leaving a steep gradient
over a narrow strip near the center.
In this report, we amplify and extend these observations. We
find that the applied currents cause appreciable amounts of charge
to be stored in the water, charge that can later be extracted in
the form of current flow. It appears that the previously reported
zones of high and low pH correspond, respectively, to regions
of net negative and positive charge.
Methods
The experimental setup consisted of an electrochemical cell with
electrodes positioned at either end, along with simple circuitry that
supplied voltage and allowed the readout of certain electrical variables
(Figure 1).
The experimental protocol involved two phases, charging and
discharging. During charging, the switch in Figure 1 was connected
to point 1, so the supplied current passed through the electrodes and
into the solution. The DC power-supply voltage was set somewhere
between 2.5 and 4 V. For discharging, the switch was connected to
point 2. From this moment on, the electrochemical cell itself worked
as a power supply, discharging any stored energy into a resistive
load. In some protocols, as noted, an extra pair of discharge electrodes
was added to, or substituted for, the original electrode pair.
The electrochemical cell was fabricated from a standard disposable
plastic spectrophotometer cuvette, with inner dimensions 5.4 × 1.0
* Corresponding author. E-mail: [email protected].
(1) Fried, I. The Chemistry of Electrode Processes; Academic Press: London,
1973.
(2) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and
Applications; Wiley: New York, 2001.
(3) Klimov, A.; Pollack, G. H. Langnuir 2007, 23, 11189–11895.
× 1.0 cm. The plastic material is transparent within the spectral
range up to 800 nm. The cuvette’s open end was closed by sheet
of plastic cut from a similar cuvette and held in place by silicon glue.
Two slots for plate electrodes were cut parallel to the chamber’s
opposite ends (Figure 1). For experiments where alternative pairs
of electrodes were used for discharging, a series of hole-pairs were
drilled into the top face of the cell along the midline and equidistant
from the slots (Figure 1). For early experiments, a similar cuvette,
but with dimensions 2.6 × 1.0 × 1.0 cm, was used and contained
slots to accommodate only one pair of electrodes.
The primary electrode pair, which was used for charging, and in
some experiments also for discharging, was made from a 1.4 cm ×
0.9 cm piece of 0.127 mm platinum foil. A second pair of electrodes,
used in the charge-distribution-defining experiments, consisted of
platinum wires 0.25 mm in diameter. Metal for all electrodes was
specified at 99.99% purity.
Because pure water is a poor conductor, in most experiments a
dilute salt solution (NaCl < 10 mM) was used. However, control
experiments were carried out in the absence of salt, with ultrapure
water (Type 1 by ASTM’s [American Society for Testing and
Materials] D1193-06 standard) or distilled water. Also, pH-sensitive
dye was employed for distinguishing regional pH values. For this
purpose, a universal indicator (Riedel-de Haen 36803 pH 3.0-10.0)
was used in concentrations recommended by the manufacturer, three
drops of liquid dye per 10 mL of solution, or approximately 1.5%
v/v.
In early experiments, electrical parameters were recorded using
a simple digital multimeter (TENMA, part # 72-7745 RC). When
currents were too small to resolve, especially during discharging,
they were calculated from the value of Rload and the observed voltage
readings across that resistor.
In later experiments, all results were obtained using a dataacquisition device (DAQ), manufactured by National Instruments
(NI USB-6009). The DAQ served both as a signal source and as
recorder of voltage and current. With software written in Labview,
the values of charging voltage, period of charging, period of discharge
recording, sampling rate, and data-file name could all be set. To
compensate for the relatively low input impedance of the DAQinput lines, high input impedance buffers were added between the
electrodes and the DAQ. Data were automatically uploaded into a
computer when the experiment was completed. For Rload, a fixed
resistor was used.
For control experiments on pH distribution during and after
electrolysis, the setup depicted in Figure 2 was used. It consisted
of two flasks, each containing 2.5 mL of water, with an interconnecting
water bridge made of silicon tubing with 3 mm inner diameter. To
pass current, a platinum electrode was immersed into each flask.
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Can Water Store Charge?
Langmuir, Vol. 25, No. 1, 2009 543
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Figure 1. Schematic model of experimental setup.
Figure 2. Setup for control experiments on pH distribution during and
after electrolysis.
Results
Charging Dynamics. Dynamics of charging were studied using
the setup of Figure 1. The electrolyte was either distilled water
or 10 mM NaCl, as indicated. The DC power supply was set to
4 V. Representative plots of current versus time are shown in
Figure 3.
The results shown in Figure 3 were found consistently in 12
such experiments. Typical curves of current versus time showed
three phases: (i) a rapid initial falloff; (ii) recovery, with continued
growth; and (iii) steady current flow, continuing until the power
supply was turned off. When pure water was used instead of the
NaCl electrolyte, results were qualitatively similar; however,
current magnitude was lower, and transitions between phases
were less obvious.
Several experiments were carried out with an electrochemical
cell shorter in length, 2.6 × 1.0 × 1.0 cm. With this shorter cell,
both volume and distance between electrodes were reduced. Out
of six experiments with 10 mM NaCl, the current plot was
displaced downward by approximately 0.07 mA as compared to
the standard curve plotted in Figure 3. Otherwise, results were
similar.
The plot of current versus time differed in nonlinear fashion
when the input voltage was altered beyond the standard window.
When the applied voltage was reduced to less than 1.5-2 V, the
initial phase (i) showed low current, which declined to zero and
never recovered to any detectable level. When the voltage was
increased above the standard range of 2.5-4 V, the current plot
shifted upward, while both the minimum point and the onset of
phase iii shifted rightward.
The dynamics of pH change during charging, more thoroughly
described in a previous report,3 are shown in Figure 4. Significant
changes of pH uniformity occur during the first several minutes
after initiation of current flow. Within 10 min, clear differentiation
is established between zones of high and low pH. Beyond that
time, changes are relatively minor.
Control experiments were devised to test whether the pH
separation might arise as a result of charging the dye molecules
instead of the water. In these experiments, current was passed
in the absence of dye, using the control setup of Figure 2. This
arrangement allows physical separation of zones, so that when
the dye is subsequently added, there is minimal disturbance and
the distinct zones cannot easily mix.
To carry out this experiment, flasks and bridge tubing were
filled with water. A potential difference of 4 V was applied to
the electrodes for one hour. The circuit was then opened, and
tubing and electrodes were immediately removed from the flasks.
Next, 50 µL of universal indicator was added to each vessel. The
color in one vessel corresponded to pH 8 and in the other vessel
to pH 4. This result was found consistently (n ) 4). Thus, large
pH differences are noted even without dye present during
charging. (For reference, a pH value of 4 corresponds to 10-6
protons per molecule of water.)
In Figure 4, a bubble can be seen growing in the cell’s upper
left corner. Bubble formation occurred in all experiments (n >
20), although position and growth rate were inconsistent. In most
cases, formation began during the charging phase and continued
through discharge. Characteristics of bubble formation were not
pursued in any detail, but may warrant future study.
Discharge Dynamics and Stored Charge Distribution. The
dynamics of pH distribution were visualized also during discharge.
For these experiments, the charging phase was set to 45 min.
Figure 5 shows representative examples of the pH distribution
during the period of discharge in two situations: when the circuit
between electrodes was open (top) or closed (bottom). In both
cases (n ) 3), zones of different pH remained for extended periods,
on the order of 1 h. Moreover, the “red” (positive) zone barely
changed in terms of depth of color until the other zone had
become almost neutral.
Because pH zones persist for long times, implying that the
attendant charges may be stored throughout the chamber, the
question arises: Can at least part of the charge be recovered? To
address this question, additional sets of electrodes were used to
first map the charge distribution. The setup and charging processes
were similar to those described above. During discharge, data
were collected with additional electrode pairs prepositioned at
each of a series of locations throughout the cell. These positions
were selectable along the midline of the chamber’s long side.
Hence, the distance between electrodes, as well as distance from
the original electrodes, could be regulated. During the charging
phase, the circuit between these additional electrodes was left
open, and, during discharge through these electrode pairs, the
electrode pair used for charging was also left open circuited.
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544 Langmuir, Vol. 25, No. 1, 2009
OVchinnikoVa and Pollack
Figure 3. Dynamics of charging phase: Potential difference of 4 V was applied to plate electrodes immersed in 10 mM NaCl electrolyte solution
(top) or pure water (bottom).
Figure 4. Water-pH dynamics during charging, recorded from above
the chamber. Electrodes situated along the top and bottom of each panel.
Times after current-flow initiation are indicated below. The colors of
the indicator correspond to the following values of pH: green pH 7,
orange pH 4, dark violet pH ∼10.
The results are shown in Figure 6. The voltage obtained from
electrodes positioned closer to charging electrodes was greater
than that obtained closer to the chamber’s center. This feature
applied over the short-term (Figure 6a) as well as the long-term
(Figure 6b). Because electrode pairs were positioned at fixed
points fairly widely separated, the voltage distribution could not
be determined with high spatial resolution. Nevertheless, the
correlation between discharge voltage and position was consistent
in 15 experiments. Although the apparent charge stored near the
center was less than that stored near the ends (near the charging
electrodes), it is notable that some charge was stored throughout
the full extent of the chamber.
To ensure that the results were not an artifact stemming from
residual charge stored or collected on the charging electrodes,
control experiments were carried out. The strategy was to remove
charging electrodes after charging and place the discharge
electrodes in the chamber only after their removal, 10 min after
charging was complete.
Figure 7 shows the results of three separate experiments along
these lines. Plot (1) is the reference curve, with both charging
and discharging electrode pairs in place throughout. Plots (2)
and (3) show data obtained from additional electrodes added 10
min after charging had ended, the latter obtained with charging
electrodes removed as the discharging electrodes were added.
The results demonstrate substantial potential difference between
added electrodes even after the charging electrodes had been
removed. The gap between curve (1) and the others may have
arisen from the disturbance attendant with electrode additions/
replacements or from the charge that may have accumulated in
discharge electrodes during the charging process. Or, because
electrode area may play a role, it is possible that the discharge
voltage might have been greater if plates had been used instead
of wires, for the additional electrodes. At any rate, substantial
charge is evidently stored throughout the chamber.
Charge Recovery. From experiments such as those illustrated
in Figure 6, it is possible to calculate the ratio of recovered
charge to input charge or energy-recovery efficiency. Because
the discharge current in the protocols above was considerably
smaller than the charge current, we decided to explore efficiency
on a shorter charge-time scale.
Figure 8 shows representative results obtained with a single
pair of plate electrodes. The system had been charged for just
3 s; however, the potential difference between electrodes persisted
for more than 2 min following power-supply disconnection. From
such records, one can integrate the current over time to obtain
the charge recovered. Similarly, integration of input current yields
input charge, and the ratio of the former to the latter yields
efficiency.
To explore the domain of efficiency, the field of parameters
that varied is evidently vast. Over the relatively limited set of
conditions explored, with plate-charging electrodes, we found
that the maximum efficiency occurred with the following
parameter values: charging time, 1 s; charging voltage, 2.5 V;
load resistance, 0.75 kΩ; solution composition, 10 mM NaCl.
Under those conditions the recovered charge was 0.14 ( 0.01
mC, while the input charge was 0.20 ( 0.01 mC (n ) 5).
Consequently, the recovery ratio was approximately 70%. This
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Can Water Store Charge?
Langmuir, Vol. 25, No. 1, 2009 545
Figure 5. pH dynamics during discharge, which began at time 0. Top: Open circuit between electrodes. Bottom: Short circuit between electrodes.
Figure 6. Time course of discharge voltage at various electrode positions. Second member of electrode pair placed symmetrically on right side. 4
V charging voltage; 1 h charging time; 10 kΩ load resistance. 10 mM NaCl electrolyte. (a) Initial 10 s from beginning of discharge; (b) full extent
of discharge, up to 30 min.
is not necessarily the absolute maximum, but more likely a local
maximum realized over the limited domain of conditions explored.
The ratio did not depend strongly on whether the solution was
pure water or water with modest salinity. A representative result
is shown in Table 1, obtained under conditions only slightly
different from those described above: single pair of wire electrodes
used for both charging and discharging; charging voltage 2.5 V;
charging time 3 s; load resistance 10 kΩ. The table shows that,
although the absolute values of charge increased by approximately
three times in the presence of salt, the ratio remained approximately the same.
Use of plate electrodes instead of wire electrodes improved
the ratio considerably. Under standard conditions (2.5 V charging
voltage for 3 s, 10 kΩ load resistance, 10 mM NaCl), the amount
of charge delivered to the cell was about the same with wire or
plate electrodes, 0.26 ( 0.01 mC (n ) 5), but the charge that
could be extracted was approximately three times higher with
the plates than with similarly positioned wires, 0.17 ( 0.01 mC
versus 0.06 ( 0.01 mC.
The presence of pH-sensitive dye also made some difference.
When the salt solution contained pH-dependent dye at the
concentration recommended by manufacturer, input and output
charges were, respectively, 0.24 ( 0.01 and 0.08 ( 0.01 mC (n
) 5), a ratio of about 0.32. The same conditions, but without the
dye, gave 0.23. Hence, the presence of dye had a modest effect
on the results.
Discussion
The present experiments were carried out to explore the
possibility of charge storage in, and subsequent recovery from,
water. Previous studies demonstrated long-term charge separation
following electrolysis,1 and this possibility had been noted in
other studies.4,5 Charge storage is also dramatically implied by
the classic Kelvin Water-Dropper experiment, an excellent
manifestation of which is presented at http://www.geekarmy.com/
Science/Cool-Battery-Demonstration.html. Droplets accumulating in proximate metallic chambers build potential difference
large enough to create visible discharge between chambers,
implying appreciable charge storage in each body of water.
What is especially surprising in the current results is that the
separation of charge within the chamber could be maintained for
long periods of time. Extensive zones of distinct pH remained
in place for up to several hours following cessation of charging
(Figure 5). Because these distinct zones correspond to zones of
positive and negative charge (Figure 6), the expectation is that
(4) Macounova, K.; Cabrera, C. R.; Holl, M. R.; Yager, P. Anal. Chem. 2000,
72, 3745–3751.
(5) Cabrera, C. R.; Finlayson, B.; Yager, P. Anal. Chem. 2001, 73, 658–666.
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546 Langmuir, Vol. 25, No. 1, 2009
OVchinnikoVa and Pollack
Figure 7. Control experiments. 4 V charge voltage, 1 h charge time, 10 kΩ load resistance. 10 mM NaCl electrolyte. Discharge electrodes located
9 mm from the charging electrodes (third hole).
Figure 8. Discharge-voltage dynamics measured after disconnection from DC power supply. Charging voltage 2.5 V, applied for 3 s; load resistance,
10 kΩ.
Table 1. Recovery Ratio (n ) 5) with and without Salt
electrolyte composition
input charge (mC)
recovered charge (mC)
recovery ratio (output/input)
10 mM NaCl
0.26 (0.01
0.06 ( 0.01
0.23
pure H2O
0.08 ( 0.01
0.02 ( 0.01
0.25
they should readily mix, but they do not neutralize easily. The
situation is much like a battery in which charges can remain
separated for long periods and can then be drawn on demand.
Electrode-surface effects are well recognized.6 However, the
separated charges appear to be stored in the water itself and not
on the electrode surfaces. This was determined by removing the
charging electrodes and replacing them with fresh electrodes:
the potential difference between half-chambers remained evident
(Figure 7). Although the potential difference was lower closer
to the chamber center, an appreciable potential difference
nevertheless remained even very near to the center (Figure 6).
(6) Conway, B. E. Electrochemical Supercapacitors: Scientific Fundamentals
and Technological Applications; Kluwer Academic Publishers: London, 1999.
Hence, charge is stored over each entire half-chamber and must
therefore lie within the water itself. It is apparently not an
electrode-based phenomenon.
Perhaps the most significant aspect of the results is that stored
charge could be recovered. This was shown by placing a resistive
load between electrodes and drawing current through this load.
The recovered-current magnitude depended on many factors.
Although no systematic attempt was made to map the range of
recovery over the full domain of possible factors, we did find
that recovery as high as 70% of input charge was not difficult
to obtain. This implies that, as a storage medium, water is quite
effective.
A number of fundamental questions are raised by these results.
One of them is why separated charges in solution can remain
separated for periods as long as those observed. Separated positive
and negative charges experience an attractive force, which should
result in immediate movement and reasonably quick neutralization, limited by viscosity. However, the results showed that charge
separation was well maintained for lengthy periods of time. Even
if some barrier were formed between the respective charged
Can Water Store Charge?
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zones, one might have expected a huge charge accumulation
immediately across the barrier, as in a capacitor; however, the
charges were reasonably well distributed over the full extent of
the respective zones. Hence, it would appear that the separated
charges are constrained in some fashion, as though confined in
some kind of lattice, like n- and p-regions of semiconductors.
Water appears able to adopt two structural networks that have
mirror symmetry to one another. The fact that these networks
are macro phenomena deserves further study.
A second and related issue is the potential for disturbance of
these structural networks. It is now established that when water
is left standing for long periods, it develops thixotropic properties,
implying macrostructure.7 Such macrostructure is expected to
be fragile. The fact that removing and inserting electrodes did
not apparently ruin the charge-containing structure implies that,
once formed, the structural network can re-form rather readily.
This is an additional subject requiring further study.
Another significant issue is the correlation between pH
difference and charge difference. Regions of low pH had excess
positive charge, whereas regions of high pH had excess negative
(7) Vybiral, B. Water and the Cell; Pollack, G. H., Cameron, I., Wheatley, D.,
Eds.; Springer: New York, 2006; pp 299-314.
Langmuir, Vol. 25, No. 1, 2009 547
charge. If that were not the case, then the charge should not have
been so easily recoverable. The question arises whether the
correlation between pH and charge seen here might be more
general. One might speculate, for example, whether ordinary
acidic solutions, which have low pH, might contain net positive
charge, while ordinary basic solutions might contain net negative
charge.
Although the routinely invoked “law of electroneutrality”
implies that net charge should not be possible in any volume of
fluid, this “law” may not be inviolable. For example, clouds,
essentially water, contain vast regions of net charge. In the present
experiments as well, half-chambers with net charge could be
sustained for long periods. Hence, the law of electroneutrality
is not necessarily inviolable over restricted volumes, and the
possibility that acidic/basic solutions might contain net charge
should not be reflexively dismissed.
Hence, the results raise a number of questions that warrant
further pursuit. At the very least, they open the possibility that
ordinary water might have the capacity not only to store charge
but to permit effective recovery of charge. Water may well be
an unexpectedly effective charge-storage medium.
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