Molten carbonate cathodic 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 PEMFC (see Table 17.2 for identification) has the greatest potential to reach high power densities. DMFCs suffer from the high activation potential of the cathodic reduction of oxygen and anodic oxidation of methanol. MCFCs operate at 650°C and SOFCs at 1000°C, their electrolytes being, respectively, molten carbonates and solid metal oxides. Their activation overpotentials are small, but ohmic overpotentials at the... 

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]. 

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. 

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. 

---