Materials of Electrochemical Equipment, Their degradation and Corrosion

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

2. Material Choices • Metals (steels) as conventional self-supporting materials for electrodes, electrolyzer troughs, gas – pipes and bipolar plates • Ionomers for diaphragms • Polymersasinsulating materials

3. Metals • CORROSION • Mechanical wear and erosion • High temperature sintering and granule growth • High temperature surface oxidation and internal oxidation of non noble constituents

4. 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

5. Carbon A special story of its own

6. 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

7. pH-potential (Pourbaix) diagrams A diagnostic thermodynamic tool Identifying existing phases as Condition for potential passivity

10. 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.

13. 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)

14. 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)

15. anodic oxidation of active Carbon At 180o to 200oC C + 2 H2OCO2 + 4 H+ + 4 e-

16. Polymers and Ionomers Properties and deterioration

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

18. 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

19. 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)

20. 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)

22. NafionTM : Perfluorinated polyether-sulfonic acid Phase-separation: aqueous/non-aqueous

23. Anion exchange membranes are chemically less stable

24. 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

25. 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)

26. 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