Download Department of Chemistry

Fuel Cell Workshop for
Secondary School Teachers
Fabrication of a Gas Diffusion Electrode
Practical Session
Education and Manpower Bureau, HKSAR
Department of Chemistry
University of Hong Kong
Sept 2003
Fabrication of an Alkaline Air Cathode
I. Objectives
To fabricate a gas diffusion electrode using common materials and apparatus.
To verify the performance of the prepared electrode as a cathode in an aluminiumair sea water cell.
II. Gas Diffusion Electrode
A gas diffusion electrode is needed for sufficient contact and penetration of a gas
reactant into the electrode. It is also called “ a breathing electrode” because it
allows sufficient transport of a gas reactant, such as oxygen or hydrogen into the
electrode. In case of oxygen, the electrode is called “air cathode”. For a gas
absorbing electrochemical reaction, the reaction takes place at the three-phase
boundary: intersection of gas phase, liquid phase, and solid phase. For example, the
oxygen reaction in alkaline, the three phase reaction is
catalyst ( s)
O2 ( g )  4e  
 2H 2 O(l )  4OH 
The geometry of the three-phase boundary is a line and has a limited capacity for
reaction. In reality, gas molecules can dissolve at the gas-liquid interface and
diffusion to the liquid-solid interface for reaction. To reduce the diffusion path and
maximize the gas-liquid interface area, a ”thinly wetted layer” is desirable. In
addition to the supply of gas, a connected conducting solid phase is required to carry
the electrons to or away from the reaction site.
A conventional gas diffusion electrode is made of
1) high surface area carbon powder loaded with a catalyst
2) a non-wetting material, PTFE to create pores and hydrophobicity,
3) a binder to hold the loose material together, and
4) a metal mesh or foam, or carbon cloth for the current collection.
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The electrode may be composed of multiple layers. A reactive “carbon” layer with
catalyst supported in active carbon, was pasted in a current collector, nickel foam. The
hydrophobic film (Teflon layer) is on the outside surface which becomes the “air side” as shown
in figure below.
Teflon Layer
Air Side
Electrolyte Side (water side)
Reaction layer:
(3 phase boundary)
Catalyst on carbon support inside
a nickel foam
Structure of an air cathode
III. Catalyst
The air cathode catalysts for oxygen reduction are highly active, non precious electro-catalysts,
such as metals, metal oxides and pyrolyzed metal chelates, supported on high surface area
carbons. To obtain the maximum performance, different catalysts are employed in different
chemical systems, e.g. different pH. Common catalysts include Manganese(KMnO4),
Silver(AgNO3), Cobalt Spinel, CoTMPP , Pt, La0.6Ca0.4CoO3.
Below are polarization curves of air cathodes with different catalysts.
Schematic of catalyst particles
absorbed on carbon powder.
Cathode Catalysts for Alkaline Fuel Cells
For alkaline system, silver nitrate AgNO3 supported on active carbon is a good catalyst.
The graph of Polarization voltage vs.
Current density is shown. (Catalyst:
silver 5M KOH electrolyte. Hg\HgO
reference electrode. 60 °C) The air
cathode is used in alkaline aluminum air
cells with current densities of up to 150
mA/cm. A typical cell voltage is 1.28 V
at current density of 100 mA/cm2 in
KOH electrolyte.
2
Cathode Catalysts for Saline Medium
For saline electrolyte and seawater, manganese catalyst(KMnO4) is preferred.
The applications in seawater include Al-air, Zinc-air and Magnesium-air fuel cells. The
polarization voltage and current density data is shown in figure below.
12% NaCl electrolyte.
Room temperature.
If used in an aluminum-air cell with 12% NaCl electrolyte, using aluminum foil as negative
electrode, a cell generates 1.07 volts at a current density of 30mA/cm2
IV Experiment
An air cathode designed for use in alkaline or saline electrolytes for metal-air/fuel cells will be
prepared and tested in an aluminium-air cell.
Characteristics of the gas diffusion electrode:
Typical Thickness:
Average Weight:
Safety:
1.5mm
900 grams per square meter (typical)
Does not contain mercury, cadmium, lead, lithium, or other dangerous
materials.
Physical Description:
A 2-layer laminate consisting of (a) A current collector of nickel foam
with reactive carbon paste (b) A micro-porous PTFE film on the air
side.
The air cathode is mechanically robust and chemically resistant to a wide range of substances.
Instruments/Chemicals
1.
2.
3.
4.
5.
6.
7.
Watch glass or Petri dish, 10cm in OD
Voltmeter X1
Connector X 2
Mini DC motor with fan blade or LED light
Brush or wooden stick
Sprayer
25% NaCL solution in H2O (Sea water)
8. Aluminum foil & scissors
9. Carbon powder, Vulcan 72 XC
10. Catalyst (KMnO4)
11. Binder (5%PVA in H2O or paper glue)
12. Teflon emulsion(30% in H2O)
13. Poly propylene Separator or filter paper
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Procedure
(1) -----Paste Preparation (20% KMnO4)
Weigh 0.95g of active carbon (XC-72), 0.2g of KMnO4 add together with 10 mL of DI H2O, 5mL 95%
ethanol, 10 drops of 5% PVA solution, 5 drops of 1/:3 PTFE suspension, to a plastic Petri dish and
stir with a glass rod for several minutes until the slurry become paste like.
(2) -----Electrode Fabrication
Use a brush or stick to apply the well-mixed paste into the pre-cut nickel foam (4X4cm2 with a lead
tab) until both sides are covered with the black paste. Dry in the 100~2000C oven for about 5-8
minutes. Use the sprayer to spray PTFE emulsion(30% in H2O) to ONE SIDE of the electrode until it is
wet, dry in the 100~2000C oven for 10 minutes.
(3) ----- Assembly of aluminum-air fuel cell and test air cathode’s performance
Use scissors to cut 4X4cm2 aluminum foil (with a protruding lead tab) and place on a shallow Petri
dish, then cover with a pre-cut PP separator (6X6 cm2)or a filter paper, finally place your air
cathode on top of the separator, Teflon side up, (to make a “sandwich” like fuel cell), pour about
5mL 25% NaCL solution onto it (the electrolyte should flood half of the air cathode but NOT to
flood the Teflon side. Finally connect each terminal to the motor fan(or a LED ) with a voltmeter,
the fan will start turning , record the voltage change against time.
***Note:
In order to avoid electrical short-circuit between cathode and anode, the area of air cathode
must be less than the area of separator.
Air cathode
+
separator
-
Al foil
Watch glass/ Petri Dish
Voltmeter
Assembly of an
Aluminum-Air
Fuel Cell
DC motor
with fan
Fuel Cell system:
Air-cathode: catalyst—KMnO4
Anode (Fuel)
Aluminium
Electrolyte: 25% NaCl in H2O
Temperature: room temperature
Open circuit voltage(OCV): ~1.05V
Location Half Cell reactions
Anode
Voltage
Al + 4 OH-—> Al(OH)4- + 3e
-2.35
-
Cathode 3/4 O2 + 3/2 H2O + 3e—> 3OH
0.40
Overall Al + 3/2 HO + 3/4 O2 —> Al(OH)3 2.75 V
4
Catalyst Loading Effect
1.5
1.4
Air-Metal Fuel Cell Perfromance:
Catalyst percentage effect
Anode: Al Foil
Cathode catalyst: KMnO4
Electrolyte: 25% NaCL
Electrode area: 4X4 cm2
1.3
1.2
1.1
A graph showing
the effect of
catalyst loading to
the performance of
air cathode.
Cell Voltage(V)
1.0
0.9
0.8
0.7
20%
10%
5%
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
5
10
15
20
25
30
35
40
Time(min) of discharge
Time
Time elapsed
Cell voltage
remark
---------------------------------------------------End of experiment------------------------------------------
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