Water and KOH Transport in
an Alkaline Fuel Cell
H.-J. Kohnke, G. Sauer, S. Schudt, Gaskatel GmbH, Germany
E. Coyle, D. Kennedy, Dublin Institute of Technology, Ireland
J. Hamilton, University of Wisconsin-Platteville, USA
H. Schmidt-Walter, University of Applied Sciences Darmstadt, Germany
Introduction
Balance of Materials (Calculation)
Methods
The measurement instruments of a deep sea measurement
probe are to be supplied with electricity from a fuel cell.
In a fuel cell, the reaction of hydrogen and oxygen produces
water while generating electric energy and, to a small extent,
heat.
The electrolyte in an AFC is an aqueous potassium hydroxide
(KOH) solution. The reaction water is absorbed in the
electrolyte and dilutes the latter.
A circulating flow through the fuel cell refreshes the electrolyte
and takes the reaction water out of the cell. Normally, the
circulation is driven by an electric pump.
However, for a deep sea application it was not possible to use
an electric pump, because the power output of the fuel cell is
too low (only 5 W).
The idea was to run an AFC without a pump. The exchange of
KOH and water should be done by diffusion processes.
Several tests were carried out to find out how an alkaline fuel
cell reacts when there is no electrolyte circulation.
The KOH concentration was measured with the concentration
sensor Densoflex while the fuel cell was running. The end
plates of the fuel cell had to be adapted for the use of the
concentration sensor. Hence it was possible to measure the
hydroxide concentration inside the electrolyte chamber.
Furthermore, the electrolyte volume in each electrolyte
chamber was measured.
• Initial concentration: 7.0 mol/l
• Current density: 10 mA/cm²
• Reaction water transported from the hydrogen electrode to
the oxygen electrode: 85 %
• KOH transported from the oxygen electrode to the hydrogen
electrode: 99 %
Concentration Sensor Densoflex
Endplate
Endplate
Objectives
• To run an AFC without using an electrolyte pump
• Get more information about the internal transport of water
molecules, OH– ions and K+ ions
• To figure out how the electrodes and the separator affect the
mass transport inside an AFC
Electrolyte
Chamber
Hydrogen
Electrolyte
Chamber
Oxygen
Electrodes/ Separators Assembly
Chemical Reactions Background
• Hydrogen/oxygen fuel cells have the same overall reaction.
Overall reaction:
H 2 + ½ O2 à H 2 O
Results
Concentration Measurement
• The overall reaction can be split into the two partial reactions
taking place at the anode and the cathode. The type of
reaction depends on the type of electrolyte being used.
Hydrogen electrode (anode):
H2 + 2 OH– à 2 H2O + 2 e–
Hydrogen reacts with OH- to form water.
• Hydrogen electrode: Raney nickel
• Oxygen electrode: silver
• Initial concentration: 7.0 mol/l, final concentration: ~ 3.5 mol/l
• Current density: 10 mA/cm²
• Mark 1: concentration H2-side is nearly constant
• Mark 2: concentration H2-side drops down
Electrolyte Volume Measurement
Load
4e-
• Hydrogen electrode: Raney nickel
• Oxygen electrode : Silflon
• Initial concentration: 7.0 mol/l, final concentration: ~ 3.5 mol/l
• Current density: 20 mA/cm²
2e-
H2O
-
e
1
+
H2O+ /2 O2
H+
OH
H2
OH-
OH
OH++
H
-
e
O2
H2O
e
H2O
-
H2O+1/2 O2
+
H+
OH-
H2
-
e-
OH
OH+
H+
H2O
Anode (-)
Electrolyte
KOH
Separator
H2O
KOH
#314
H2O
KOH
#249
Silflon
H2O
KOH
Oxag
H2O
VE
[ml]
21.0
27.0
22.0
25.0
23.0
28.0
22.0
25.0
[ml]
22.9
24.8
18.0
28.7
24.6
26.3
26.0
20.9
cA
cE
delta n delta n
(KOH) (KOH) (KOH)
(H2O)
[mol/l] [mol/l] [mmol/h] [mmol/h]
7.000 5.081
-23
104
0.000 1.148
21
-102
6.907 6.704
-38
-238
0.000 1.161
40
227
7.180 6.355
-6
77
0.000 0.195
3
-67
7.793 6.550
-2
464
0.000 0.375
16
-462
There are three possible explanations for this behaviour:
• The concentration gradient is higher at a higher current. This
means that more water will be transported. The movement of
KOH is not influenced by the gradient; it only depends on the
electric field.
• The electrodes and separators have an influence on the
diffusion-based transport of the KOH and the water; they
transport much more water than KOH. However, the KOH
transport caused by an electric field is quite different in
comparison to the transport caused by diffusion.
• The volume flow was also observed in an alkaline
electrolyser. The movement of the electrolyte is towards the
hydrogen electrode (OH– ion-producing electrode, i.e.
cathode). A third explanation for this behaviour would be
electro-osmosis. However, this is not clear at the moment.
Future Work
2e-
OH-
Electrode/
Separator
VA
During the test the reaction water of the AFC leaves the cell at
the oxygen electrode. Only at low current densities, a part of
the water leaves at the hydrogen electrode. The concentration
of the KOH in the electrolyte chamber of the oxygen electrode
is slightly higher than the KOH concentration in the electrolyte
chamber of the hydrogen electrode.
The measured values are reached when 85 % of the reaction
water is transported to the oxygen electrode. The higher the
current density the higher the proportion of the transported
water. Also the calculation shows this correlation.
The driving force for the OH– ions is the electric field between
the electrodes, while for the water molecules it is the
concentration gradient.
The figure below shows the mass transport cycle inside an
alkaline fuel cell.
Water has to move from the anode to the cathode; OH– ions
have to move from the cathode to the anode. On their way they
have to pass parts of the electrodes and the active separator.
The active separator is placed in between the anode and the
cathode. The reaction water has to pass two electrodes and
one or two separators (depending on where the water leaves
the cell).
Mass transport cycle of an AFC
4e
• Initial concentration chamber 1 (KOH): 7 mol/l
• Initial concentration chamber 2 (H2O): de-ionised water
• cA/ VA – Concentration/ Volume at the beginning
cE/ VE – Concentration/ Volume at the end
• Negative sign: substance leaving the chamber
• Positive sign: substance entering the chamber
Discussion
Oxygen electrode (cathode):
½ O2 + H2O + 2 e– à 2 OH–
Oxygen reacts with water to form OH–.
-
Diffusion Tests
Cathode (+)
• Performing more tests on electrolyte transport caused by
electro-osmosis.
• Retrying the concentration measurement and the volume
measurement with an alkaline electrolyser and comparing the
results with the fuel cell.
• Finding out how the potential influences the contact angle
between the KOH solution and the electrode.