Transcript Materials of Electrochemical Equipment, Their degradation

Materials of Electrochemical Equipment,
Their degradation and Corrosion
Summer school on electrochemical
engineering,
Palic, Republic of Serbia
Prof. a.D. Dr. Hartmut Wendt, TUD
Material Choices
• Metals (steels) as conventional selfsupporting materials for electrodes,
electrolyzer troughs, gas – pipes and bipolar
plates
• Ionomers for diaphragms
• Polymers as insulating materials
Metals
• CORROSION
• Mechanical wear and erosion
• High temperature sintering and granule
growth
• High temperature surface oxidation and
internal oxidation of non noble constituents
Polymers and Ionomers
• Bon breaking by oxidation (oxygen and
peroxides)
• Reduction ( lower valent metal ions,
hydrogen)
• Solvolysis (preferentially hydrolysis) by
acids and bases.
• Particular for Ionomer membranes (MEAs)
is delamination
Carbon
A special story of its own
Characteristic data of some important metallic materials
Material
unalloyed steels
200 to 300

density
g/cm3
7.8
stainless steels
200 to 300

8.2
100
9.
3.8 to 4.7
titanium
420 to 650
4.5
6
zirconium
500 to 700
6.4
10
hafnium
500 to 1200

13
16.6
200 to 350
nickel
tantalum***
UTS*
N/mm2
----------------------------------------------------------------*
UTS = Ultimate tensile strength
**
Price in US $/kg; calculated from prices valid for the Ger.Fed.Rep. 1997 with rate of
1 US $ = 1.7 DM
***
very soft and ductile material which may be used only for corrosion-protection coatings
price**
US$/kg
0.5
1.5 to 3
200
exchange
pH-potential (Pourbaix) diagrams
A diagnostic thermodynamic tool
Identifying existing phases as
Condition for potential passivity
What tells the Pourbaix diagram ?
• Iron might become passive at O2 – potential
and at pH beyond 2. It will never be immune.
• Nickel is immune at pH greater 8 in presence
of hydrogen, but there is only a reserve of 80
mV
• Chromium (and steels with Cr) is never
immune but might become passive
• Titanium is never immune but might
become passive over total pH – range and
potentials more positive than RHE.
High temperatures and Metals
• High temperatures (> 600oC), and longterm
exposure in HT – fuel cells would lead to total
oxidation on oxygen side (exception is only gold).
• Fe-containing alloys might become passive
because of formation of protective oxide layers
from alloy components (W,Mo,Cr. Al and other).
• Internal oxidation by oxygen diffusion into metals
and preferential oxidation of non-noble
components can change internal structure
(dispersion hardening)
• On hydrogen side there might occur hydrogenembrittlement (Ti, Zr)
Carbon in Fuel Cells
• The element carbon is not nobler than
hydrogen.
• It is unstable against atmospheric and
anodic oxidation in particular at enhanced
temperature (PAFC: 220oC)
• At still higher temperature it also becomes
unstable towards steam (C+H20 ->CO+H2)
anodic oxidation of
active Carbon
At 180o to 200oC
C + 2 H2O  CO2 + 4 H+ + 4 e-
Polymers and Ionomers
Properties and deterioration
1
Table 4
Properties and approximate prices of some polymeric materials
polymer
abbreviation
max. temperature/°C
without creeping
highest temp./°C
for utilizing
density
g cm-3
price*
US $ / kg
polyethylene high density
PEHD
45
40
0.95
0.9
polyethylene low density
PELD
-
40
0.88
0.8
polypropylene
PP
60
55
0.91
0.9
polystyrene
PST
75
60
1.04
0.9
High density polystyrene
HDPST
polyvinylchloride
PVC
75
60
1.40
0.64
poly-fluoroethylene- propylene
FEP
105
120
2.1
3
poly-perfluoroalkyl-vinylether
PFA
160
200
2.1
4
polytetrafluoroethylene
PTFE
160
220
2.2
 4.5
polyarylethersulfone+
PS
180
120
1.2
*
1.0
Price in Germany mid 1997. Rate of exchange: 1 US $ equal 1.7 DM, Source: Kunststoff Information (KI), D - 61350 Bad Homburg
Source: AMOCO
* Price in Germany mid 1997. Rate of exchange: 1 US $ equal 1.7 DM, Source: Kunststoff Information (KI), D - 61350 Bad Homburg
+
Non – Fluorinated Polymers
• May only be used with non – oxidizing
electrolytes and atmospheres
• Very often need glass-fiber enforcement
• Chlorinated and perchlorinated polymers are
chemically more stable than non-chlorinated
polymers
• Polyesters and amides are sensitive against
hydrolysis in strongly acid and caustic electrolyte
• They are cheaper than fluorinated polymers
Polystyrenes are not acceptable for Fuel cells and electrolyzers
Fluorinated Polymers
• Perfluorinated Polymers (TeflonTM) are
most stable polymers
• They are soft and tend to creep and flow
• Polyvinyliden-fluoride tends to stresscorrosion-cracking at elevated temperature
in contact to acid soltutions
(For details look at DECHEMA- WERKSTOFFTABELLEN)
Ionomers – Ion-exchange membranes
• In batteries non-fluorinated ion-exchange
membranes are sometimes used as
separators – but are usually too expensive
• NafionTM had been developed for the cloroalkali electroysis and had become the
material of choice for fuel cells (PEMFC)
• Weakness: High water transfer; at least
4H2O per H+ transferred (also methanol)
NafionTM : Perfluorinated polyether-sulfonic acid
Phase-separation: aqueous/non-aqueous
Ion exchange membranes
Commercial Name
Manufactor
NeoSepta CM 1,2,X*
NeoSepta AM 1,3,X*
Nafion
Nafion NE-455
Tokyama soda
Tokyama soda
Dupont
Dupont
Type
perfluorinated cation exchange
perfluorinated anion exchange
perfluorinated cation exchange
perfluorinated cation exchange 97 %
current efficiency at 33 % KOH
*
Flemion
Asahi Glass
perfluorinated strongly acidic cation exchange and strongly basic anion exchange
Selemion*
Asahi Glass
chemically particularly stabilized,
highest permselectivity
*
Gore Select
W.L. Gore Ass. perfluorinated cation exchange
reinforced by PTFE fabric
*
FuMA-Tech membranes FuMA-Tech
anion and cation exchange, particularly
tailored to customers demand
* Costs depend on customers demands, technological purpose and the amount ordered
Anion exchange membranes are chemically less stable
-
-
-
Delamination of MEAs
• Reason: Weak contact between
prefabricated PEM and PEM-bonded
elctrocatalyst layer
• Lifetime of MEAs can be extended steady
fuel cell operation, because repeated
hydration/dehydration with subsequent
change of degree of swelling exerts stress
on the bond between membrane and catalyst
NEW membrane materials
• Aim: reduce swelling, water and methanol
or ethanol transport, improve durability of
contact between membrane and catalyst
layer
• Sulfonated polyaryls, polyethetherketones
(PEEKs) and Polyaryl-sulfones (all new
PEM-materials are sulfonic acids)
Summary
The electrochemical engineer needs
not to be an expert in material science
but he needs to know when to go and
ask material scientists