Materials of Electrochemical Equipment, Their degradation and Corrosion Summer school on electrochemical engineering, Palic, Republic of Serbia Prof. a.D.

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 self-supporting 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 UTS* density price**N/mm2 g/cm3 US$/kg

unalloyed steels 200 to 300 7.8 0.5 stainless steels 200 to 300 8.2 1.5 to 3 nickel 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 13 200 tantalum*** 16.6 200 to 350 -----------------------------------------------------------------* UTS = Ultimate tensile strength** Price in US $/kg; calculated from prices valid for the Ger.Fed.Rep. 1997 with rate of exchange 1 US $ = 1.7 DM*** very soft and ductile material which may be used only for corrosion-protection coatings

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 hydrogen-embrittlement (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

Price in Germany mid 1997. Rate of exchange: 1 US $ equal 1.7 DM, Source: Kunststoff Information (KI), D - 61350 Bad Homburg

1

Table 4 Properties and approximate prices of some polymeric materials

polymer abbreviation max. temperature/°C highest temp./°C density price* without creeping for utilizing g cm-3 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 1.0

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

* Price in Germany mid 1997. Rate of exchange: 1 US $ equal 1.7 DM, Source: Kunststoff Information (KI), D - 61350 Bad Homburg + Source: AMOCO

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 polymersPolystyrenes 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 stress-corrosion-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 cloro-alkali electroysis and had become the material of choice for fuel cells (PEMFC)

• Weakness: High water transfer; at least 4H2O per H+ transferred (also methanol)

Phase-separation: aqueous/non-aqueous

NafionTMTM : Perfluorinated polyether-sulfonic acid : Perfluorinated polyether-sulfonic acid

Ion exchange membranes

Commercial Name Manufactor Type

NeoSepta CM 1,2,X* Tokyama soda perfluorinated cation exchange - NeoSepta AM 1,3,X* Tokyama soda perfluorinated anion exchange - Nafion Dupont perfluorinated cation exchange Nafion NE-455 Dupont perfluorinated cation exchange 97 % current efficiency at 33 % KOH - Flemion

* Asahi Glass perfluorinated strongly acidic cation ex- change 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