The Phosphoric Acid Fuel Cell (PAFC) power plants are approaching the stage of practical use. During the development of PAFC power plants, however, several major problems arose and great effort was needed to solve them. The biggest problem was unusual corrosion of the cathode carbon. In order to make clear the mechanism, authors investigated the potential behavior in the horizontal plane of a cell using a new electrochemical technique (a single cell with multi-reference electrodes).

Under starving conditions of fuel, the cathode potential at the fuel outlet area shifted significantly toward positive, and cathode corrosion was observed at that area. Remarkable potential shift and carbon corrosion were also observed at the fuel downstream of a matrix pinhole. The cathode potential at that place exceeded 1.4 V vs.RHE. The changes of the potential shift always coincided with the changes of the RHE potentials in the horizontal plane of a cell. The potential shift caused by the local acidity change, which comes from the resistance of ions in the plane of a matrix, is the origin of the unusual cathode carbon corrosion.

As a countermeasure of the unusual cathode corrosion, authors succeeded to decrease the potential shift significantly by application of series fuel gas flow instead of parallel fuel gas flow.