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

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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