Molten oxygen reduction

Janowitz, K. Kah, M. Wendt, H. Molten carbonate fuel cell research part I. Comparing cathodic oxygen reduction in lithium/potassium and lithium/sodium carbonate melts. Electrochim. Acta 1999, 45 (7), 1025-1037. 

The oxygen reduction reaction is not straightforward and proceeds via either the peroxide ion (02 ) or the superoxide ion (02 ), as dictated by the electrolyte composition. These ions can dissolve in the molten carbonate and corrode the counter electrode. The electrolyte itself is also an aggressive medium. 

The high temperature improves the oxygen reduction kinetics dramatically eliminating the need for precious metal catalysts. The molten carbonate (usually a Li-K or Li-Na carbonate) is stabilized in a matrix (LiA102) that can be supported with AI2O3 fibers for mechanical strength. 

Lu S H and Selman J R (1992), Electrode kinetics of oxygen reduction in molten carbonate ,/ Etecfroana/ Chem,333,257-271. 

There exist a variety of fuel cells. For practical reasons, fuel cells are classified by the type of electrolyte employed. The following names and abbreviations are frequently used in publications alkaline fuel cells (AFC), molten carbonate fuel cells (MCFC), phosphoric acid fuel cells (PAFC), solid oxide fuel cells (SOFC), and proton exchange membrane fuel cells (PEMFC). Among different types of fuel cells under development today, the PEMFC, also called polymer electrolyte membrane fuel cells (PEFC), is considered as a potential future power source due to its unique characteristics [1-3]. The PEMFC consists of an anode where hydrogen oxidation takes place, a cathode where oxygen reduction occurs, and an electrolyte membrane that permits the transfer of protons from anode to cathode. PEMFC operates at low temperature that allows rapid start-up. Furthermore, with the absence of corrosive cell constituents, the use of the exotic materials required in other fuel cell types is not required [4]. 

Makkus, R.C. (1991) Electrochemical Studies on the Oxygen Reduction and NiO(Li) Dissolution in Molten Carbonate Fuel Cells. Dissertation, Delft University of Technology. 

Two parts are treated one is the physical and chemical features of materials of molten carbonate fuel cells (MCFCs), and the other is performance analysis with a 100 cm class single cell. The characteristics of the fuel cell are determined by the electrolyte. The chemical and physical properties of the electrolyte with respect to gas solubility, ionic conductivity, dissolution of cathode material, corrosion, and electrolyte loss in the real cell are introduced. The reactirm characteristics of hydrogen oxidation in molten carbonates and materials for the anode of the MCFC are reviewed. The kinetics of the oxygen reduction reaction in the molten carbonates and state of the art of cathode materials are also described. Based on the reaction kinetics of electrodes, a performance analysis of MCFCs is introduced. The performance analysis has importance with respect to the increase in performance through material development and the extension of cell life by cell development. Conventional as well as relatively new analysis methods are introduced. 

Nishina T, Uchida I, Selman JR (1994) Gas electrode reactions in molten carbonate media V. Electrochemical analysis of the oxygen reduction mechanism at a fully immersed gold electrode. J Electrochem Soc 141 1191-1198... 

Uchida I, Mugikura Y, Nishina T, Itaya K (1986) Gas electrode reactions in molten carbonate media II. Oxygen reduction kinetics on conductive oxide electrodes in (Li -t- K)C03 eutectic at 650 °C. J Electroanal Chem 206 241-252... 

Lee CG, Yamada K, Hisamitsu Y, Ono Y, Uchida I (1999) Kinetics of oxygen reduction in molten carbonates under pressurized Air/C02 oxidant gas conditions. Electrochemistry... 

Schatifker BR, Zelenay P, Bockris JOM. The kinetics of oxygen reduction in molten phosphoric acid at high temperatures. J Electrochem Soc 1977 134 2714-25. 

Cathodes for MCFCs are usually NiO made by an anodic oxidation of a Ni sinter or by an in situ oxidation of Ni metal during the cell start-up time [18,20]. NiO cathodes are active enough for oxygen reduction at high temperatures, so a Pt-based metal is not necessary. A problem with the NiO cathode occurs as over time the NiO particles grow as they creep into the molten carbonate melt that reduces the active surface area and can cause short-circuiting of the cell. One of the solutions for this problem is the addition of small amounts of magnesium metal to the cathode and the electrolyte for stability. Also, the use of a different electrolyte that decreases the dissolution of the NiO cathode is possible. 

Impurities can be removed by formation of a gaseous compound, as in the fire-refining of copper (qv). Sulfur is removed from the molten metal by oxidation with air and evolution of sulfur dioxide. Oxygen is then removed by reduction with C, CO, in the form of natural gas, reformed... 

Mg ribbon and fine Mg shavings can be ignited at air temps of about 950°F (Ref 26). Oxides of Be, Cd, Hg, Mo and Zn can react explosively with Mg when heated (Ref 8). Mg reacts with incandescence when heated with the cyanides of Cd, Co, Cu,Pb, Ni or Zn or with Ca carbide (Ref 9). It is spontaneously flam-mable when exposed to moist chlorine (Ref 10), and on contact with chloroform, methyl chloride (or mixts of both), an expl occurs (Ref 4). Mg also reacts violently with chlorinated hydrocarbons, nitrogen tetroxide and A1 chloride (Ref 14). The reduction of heated cupric oxide by admixed Mg is accompanied by incandescence and an expin (Ref 7).Mg exposed to moist fluorine is spontaneously flammable (Ref 11). When a mixt of Mg and Ca carbonate is heated in a current of hydrogen, a violent ex pin occurs (Ref 12). When Mo trioxide is heated with molten Mg, a violent detonation occurs (Ref 1). Liq oxygen (LOX) gives a detonable mixt when... 

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