Cathodic poison

Underpotential deposition of heavy metals on H2 evolving electrodes is a well known problem [133], The existence of a direct correlation between H2 evolution activity and metal work function, makes UPD very likely on high work function electrodes like Pt or Ni. Cathode poisoning for H2 evolution is aggravated by UPD for two reasons. First, deposition potentials of UPD metals are shifted to more anodic values (by definition), and second, UPD favors a monolayer by monolayer growth causing a complete coverage of the cathode [100]. Thus H2 evolution may be poisoned by one monolayer of cadmium for example, the reversible bulk deposition potential of which is cathodic to the H2 evolution potential. 

Anodic corrosion in case of platinum metals mostly is insignificant or at least small for most anolyte compositions and conditions. But it may be an economic problem for industrial applications. Furthermore, as aforementioned, it can be the reason of cathode poisoning. The corrosion rate of gold, and especially of the less noble metals, is very dependent on the pH value of the anolyte. 

MCFC - <0.5 ppm sulfur as H2S (at the cathode) equates to <10 ppm at the anode because of fuel exhaust being sent to the cathode in an MCFC system (same amount of sulfur, more gas at the cathode), poisoning is reversible. 

B. Baranowski, M. Smialowski, Charging of nickel films with hydrogen evolved electrolyti-cally in the presence of cathodic poisons, J. Phys. Chem. Sol. 12 (1960) 206-207. 

Hydrogen embrittlement can occur in carbon and low-alloy steels, in ferritic and martensitic stainless steels, and in duplex stainless steels. It is normally not a problem in either the austenitic stainless steels or nickel-based high alloys. Hydrogen can dissolve in a steel as a result of a number of phenomena (1) Corrosion creates nascent hydrogen, usually in the presence of a cathodic poison. 

H (evolved) and this is likely to be very sensitive to cathodic poisons in the solution and to certain elements within the alloy. Cracking in Al and Ti alloys and in high strength steels is accelerated by the presence of cathodic poisons. This is very strong evidence that H plays a role in the cracking process. 

Another cathodic inhibitor that is used in practice is AS2O3, which is often referred to as cathodic poison. 

Finally, under prolonged shutdown conditions with the cell continually provided with fuel, both, the alcohol and oxygen permeate the membrane. These effects generate a transient condition in which fuel exists in both electrodes resulting in a reversal current, cathode poisoning and anode catalyst oxidation [100]. 

Cathodic poisons are substances that interfere with the cathodic reduction reactions, i.e., hydrogen atom formation and hydrogen gas evolution. The rate of the cathodic reaction is slowed, and because anodic and cathodic reactions must proceed at the same rate, the whole corrosion process is slowed. 

While some cathodic poisons such as sulfides and selenides are adsorbed on the metal surface, compounds of arsenic, bismuth, and antimony are reduced at the cathode to deposit a layer of the respective metals. Sulfides and selenides generally are not useful inhibitors because they are not very soluble in acidic solutions, they precipitate many metal ions, and they are toxic. Arsenates are used to inhibit corrosion in strong acids, but in recent years the trend has been to rely more on organic inhibitors because of the toxicity of arsenic. 

In spite of having many favorable characteristics, the metallic interconnects also suffer fi-om certain drawbacks. Some of the pertinent issues are electrical contacts between metallic interconnect and ceramic electrodes (cathode and anode), matching of thermal expansion between the metallic interconnect and adjacent components, oxide scale formation on the metallic surface as well as cathode poisoning. All these issues need drastic improvement. For more than a decade, a number of alloys have been attempted, but the major interest for development of such metallic interconnect started only when SOFC developers started using metallic interconnects for SOFC operation, preferably < 750°C. 

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