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